Review of current progress in hole-transporting materials for perovskite solar cells

Recent advancements in perovskites’ application as a solar energy harvester have been astonishing. The power conversion efficiency(PCE) of perovskite solar cells(PSCs) is currently reaching parity(>25 percent), an accomplishment attained over past decades. PSCs are seen as perovskites sandwiched between an electron transporting material(ETM) and a hole transporting material(HTM). As a primary component of PSCs, HTM has been shown to have a considerable effect on solar energy harvesting, carrier extraction and transport, crystallization of perovskite, stability, and price. In PSCs, it is still necessary to use a HTM.While perovskites are capable of conducting holes, they are present in trace amounts, necessitating the use of an HTM layer for efficient charge extraction. In this review, we provide an understanding of the significant forms of HTM accessible(inorganic, polymeric and small molecule-based HTMs), to motivate further research and development of such materials. The identification of additional criteria suggests a significant challenge to high stability and affordability in PSC.

Recent advances of Cu-based hole transport materials and their interface engineering concerning different processing methods in perovskite solar cells

In recent years, perovskite solar cells (PSCs) have become a much charming photovoltaic technology and have triggered enormous studies worldwide, owing to their high efficiency, low cost and ease of preparation. The power conversion efficiency has rapidly increased by more than 6 times to the current 25.5% in the past decade. Hole transport materials (HTMs) are an indispensable part of PSCs, which great affect the efficiency, the cost and the stability of PSCs. Inorganic Cu-based p-type semiconductors are a kind of representative inorganic HTMs in PSCs due to their unique advantages of rich variety, low cost, excellent hole mobility, adjustable energy levels, good stability, low temperature and scalable processing ability. In this review, the research progress in new materials and the control of photoelectric properties of Cu-based inorganic HTMs were first summarized systematically. And then, concerning different processing methods, advances of the interface engineering of Cu-based hole transport layers (HTLs) in PSCs were detailly discussed. Finally, the challenges and future trends of Cu-based inorganic HTMs and their interface engineering in PSCs were analyzed.

Improving the performance of arylamine-based hole transporting materials in perovskite solar cells： Extending π-conjugation length or increasing the number of side groups？

In this work, we prepared three simple arylamine-based hole transporting materials from commercially available starting materials. The effect of extending z-conjugation length or increasing the number of side groups compared with reference compound on the photophysical, electrochemical, hole mobility properties and performance in perovskite solar cells were further studied. It is noted that these two kinds of molecular modifications can significantly lower the HOMO level and improve the hole mobility, thus improving the hole injection from valence band of perovskite. On the other hand, the compound with more side groups showed higher hole injection efficiency due to lower HOMO level and higher hole mo- bility compared with the compound with extending π-conjugation length. The perovskite solar cells with the modified molecules as hole transporting materials showed a higher efficiency of 15.40% and 16.95%, respectively, which is better than that of the reference compound （13.18%）. Moreover, the compound with increasing number of side groups based devices showed comparable photovoltaic performance with that of conventional spiro-OMeTAD （16.87%）.

Highly efficient perovskite solar cells based on symmetric hole transport material constructed with indaceno[1,2-b:5,6-b’]dithiophene core building block

Two novel hole transport materials (HTMs) with indaceno[1,2-b:5,6-b’]dithiophene (IDT) as core building blocks,termed IDT1 and IDT2,were designed and synthesized.The side alkyl chains were introduced to regulate and control the morphology and stacking behavior of HTMs,and the peripheral triarylamine arms were introduced to adjust the energy levels and to facilitate efficient hole transport.Applied in mesoporous structured perovskite solar cells (PSCs),HTM IDT1 achieved higher power conversion efficiency (PCE,19.55%) and better stability than Spiro-OMeTAD (19.25%) and IDT2 (15.77%) based PSC.These results suggest the potential of IDTl as a promising HTM for PSCs.

Diketopyrrolopyrrole based D-π-A-π-D type small organic molecules as hole transporting materials for perovskite solar cells

Three novel diketopyrrolopyrrole （DPP） based small organic molecules were synthesized as hole transporting materials for perovskite solar cells. The effects of different donors and zr bridges on the performance of perovskite solar cells （PSCs） were discussed. The efficiency of TPADPP-1, TPADPP-2. PTZDPP-2 was 5.10%, 9.85% and 8.16% respectively. Compared to TPADPP-2, the voltage of PTZDPP-2 was higher. Because the electron-donatingability of phenothiazine based donor was larger than that of triphenylamine based donor, the HOMO level of PTZDPP-2 was lower than that of TPADPP-2. The results indicated that the diketopyrrolopyrrole based D-π-A-π-D type small organic molecule might be a promising hole trans- porting material in the perovskite solar cells.

Progress in hole-transporting materials for perovskite solar cells

In recent years the photovoltaic community has witnessed the unprecedented development of perovskite solar cells（PSCs） as they have taken the lead in emergent photovoltaic technologies. The power conversion efficiency of this new class of solar cells has been increased to a point where they are beginning to compete with more established technologies. Although PSCs have evolved a variety of structures, the use of hole-transporting materials（HTMs） remains indispensable. Here, an overview of the various types of available HTMs is presented. This includes organic and inorganic HTMs and is presented alongside recent progress in associated aspects of PSCs, including device architectures and fabrication techniques to produce high-quality perovskite films. The structure, electrochemistry, and physical properties of a variety of HTMs are discussed, highlighting considerations for those designing new HTMs. Finally, an outlook is presented to provide more concrete direction for the development and optimization of HTMs for highefficiency PSCs.

A novel phenoxazine-based hole transport material for efficient perovskite solar cell

Based on the previous research work in our laboratory, we have designed and synthesized a small-molecule, hole transport material （HTM） POZ6-2 using phenoxazine （POZ） as central unit and dicyanovinyl units as electron-withdrawing terminal groups. Through the introduction ofa 2-ethyl-hexyl bulky chain into the POZ core unit, POZ6-2 exhibits good solubility in organic solvents. In addition, POZ6-2 possesses appropriate energy levels in combination with a high hole mobility and conductivity in its pristine form. Therefore, it can readily be used as a dopant-flee HTM in perovskite solar cells （PSCs） and a conversion efficiency of 10.3% was obtained. The conductivity of the POZ6-2 layer can be markedly enhanced via doping in combination with typical additives, such as 4-tert-butylpyridine （TBP） and lithium bis（trifluoromethanesulfonyl） imide （LiTFS1）. Correspondingly, the efficiency of the PSCs was further improved to 12.3% using doping strategies. Under the same conditions, reference devices based on the well-known HTM Spiro-OMeTAD show an efficiency of 12.8%.

Heteroatom engineering on spiro-type hole transporting materials for perovskite solar cells

A series of spiro-type hole transporting materials, spiro-OMe TAD, spiro-SMe TAD and spiro-OSMe TAD,with methoxy, methylsulfanyl or half methoxy and half methylsulfanyl terminal groups are designed and prepared. The impact of varied terminal groups on bulk properties, such as photophysical, electrochemical, thermal, hole extraction, and photovoltaic performance in perovskite solar cells is investigated.It is noted that the terminal groups of the hole transporting material with half methoxy and half methylsulfanyl exhibit a better device performance and decreased hysteresis compared with all methoxy or methylsulfanyl counterparts due to better film-forming ability and improved hole extraction capability.Promisingly, the spiro-OSMe TAD also shows comparable performance than high-purity commercial spiro-OMe TAD. Moreover, the highest power conversion efficiency of the optimized device employing spiro-OSMe TAD exceeding 20% has been achieved.

Decorating hole transport material with −CF_(3) groups for highly efficient and stable perovskite solar cells

The hole transport material (HTM) plays an extremely important role to determine the power conversion efficiency (PCE) and the stability of perovskite solar cells (PSCs). Herein, we report an effective strategy to improve the performance of HTMs by introducing −CF_(3) groups via the rational decorative mode. Upon direct attachment or nonconjugated alkoxyl bridging of −CF_(3) groups on the terminal diphenylamines, the resulting molecular HTMs, i.e., 2,7-BCzA4CF_(3) and 2,7-BCzA4OCCF_(3), show distinct properties. Compared with 2,7-BCzA4CF_(3), the nonconjugated alkoxyl bridging −CF_(3) group-based 2,7-BCzA4OCCF_(3) exhibits better thermal stability, hydrophobicity, and a dramatically upgraded hole mobility by 135.7-fold of magnitude to 1.71 × 10^(−4) cm^(2) V^(−1) S^(−1). The PSCs with 2,7-BCzA4OCCF_(3) as HTM exhibit an PCE of up to 20.53% and excellent long-term stability, maintaining 92.57% of their performance for 30 days in air with humidity of 30% without encapsulation. This work provides beneficial guidelines for the design of new HTMs for efficient and stable PSCs.

C≡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,C≡N-based molecules have been proved as efficient HTMs.Herein,a series of novel C≡N functionalized carbazole-arylamine derivatives with variable C≡N 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 Ctriple bondN 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 C≡N 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 C≡N 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 C≡N 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 C≡N unit on 2m-HTM.The results give some new insights for designing ideal HTM for efficient and stable PSCs.

Highly stable perovskite solar cells with a novel Ni-based metal organic complex as dopant-free hole-transporting material

Hole-transporting material(HTM)plays a paramount role in enhancing the photovltaic performance of perovskite solar cells(PSCs).Currently,the vast majority of these HTMs employed in PSCs are organic small molecules and polymers,yet the use of organic metal complexes in PSCs applications remains less explored.To date,most of reported HTMs require additional chemical additives(e.g.Li-TFSI,t-TBP)towards high performance,however,the introduction of additives decrease the PSCs device stability.Herein,an organic metal complex(Ni-TPA)is first developed as a dopant-free HTM applied in PSCs for its facile synthesis and efficient hole extract/transfer ability.Consequently,the dopant-free Ni-TPAbased device achieves a champion efficiency of 17.89%,which is superior to that of pristine Spiro-OMeTAD(14.25%).Furthermore,we introduce a double HTM layer with a graded energy bandgap containing a Ni-TPA layer and a CuSCN layer into PSCs,the non-encapsulated PSCs based on the Ni-TPA/CuSCN layers affords impressive efficiency up to 20.39%and maintains 96%of the initial PCE after 1000 h at a relative humidity around 40%.The results have demonstrated that metal organic complexes represent a great promise for designing new dopant-free HTMs towards highly stable PSCs.

Enhanced Photovoltage for Inverted Perovskite Solar Cells Using Delafossite CuCrO_(2) Hole Transport Material

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)has been widely adopted as hole transport material(HTM)in inverted perovskite solar cells(PSCs),due to high optical transparency,good mechanical flexibility,and high thermal stability;however,its acidity and hygroscopicity inevitably hamper the long-term stability of the PSCs and its energy level does not match well with perovskite materials with a relatively low open-circuit voltage.In this work,p-type delafossite CuCrO_(2)nanoparticles synthesized through hydrothermal method was employed as an alternative HTM for triple cation perovskite[(FAPbI_(3))_(0.87)(MAPbBr_(3))_(0.13)]_(0.92)(CsPbI_(3))_(0.08)(possessing better photovoltaic performance and stability than conventional CH3NH3PbI3)based inverted PSCs.The average open-circuit voltage of PSCs increases from 908 mV of the devices with PEDOT:PSS HTM to 1020 m V of the devices with CuCrO_(2)HTM.Ultraviolet photoemission spectroscopy demonstrates the energy band alignment between CuCrO_(2)and perovskite is better than that between PEDOT:PSS and perovskite,the electrochemical impedance spectroscopy indicates CuCrO_(2)-based PSCs exhibit larger recombination resistance and longer charge carrier lifetime than PEDOT:PSS-based PSCs,which contributes to the high VOCof CuCrO_(2)HTM-based PSCs.

Pyridine-triphenylamine hole transport material for inverted perovskite solar cells

In the light of superior interaction between pyridine unit and perovskite,a facile star-shaped triphenylamine-based hole transport material(HTM)incorporating pyridine core(coded as H-Pyr)is designed and synthesized.A reference HTM with benzene core,coded as H-Ben,is also prepared for a comparative study.The effects of varying core on HTMs are investigated by comparing the photophysical,electrochemical and hole mobility properties.It is found that pyridine core exhibits better conjunction and decreased dihedral angles with triphenylamine side arms than that of benzene,leading to obviously better hole mobility and well-matched work function.The perovskite film prepared on H-Pyr also shows improved crystallization than on H-Ben.Photoluminescence and electrochemical impedance studies indicate improved charge extraction and reduced recombination in the H-Pyr-based perovskite solar cells.Consequently,H-Pyr-based device exhibits higher efficiency than H-Ben-based one.After doping with a Lewis acid,tris(pentafluorophenyl)borane,H-Pyr-based device delivers a champion efficiency of 17.09%,which is much higher compared with 12.14% of the device employing conventional poly(3,4-ethy lenedioxythiophene)polystyrene sulfonate(PEDOT:PSS)as HTM.Moreover,the H-Pyr-based device displays good long-term stability that the power conversion efficiency remains over 80% of the initial value after storage in ambient(relative humidity=50±5%)for 20 days.

Titanylphthalocyanine as hole transporting material for perovskite solar cells

Titanylphthalocyanine （TiOPc） as hole transporting material （HTM） was successfully synthesized by a simple process with low cost. Perovskite solar cells using the TiOPc as HTM were fabricated and characterized. TiOPc as HTM plays an important role in increasing the power conversion efficiency （PCE） by minimizing recombi- nation losses at the perovskite/Au interface because TiOPc as HTM can extract photogenerated holes from the perovskite and then transport quickly these charges to the back metal electrode. In the research, the β-TiOPc gives a higher PCE than α-TiOPc for the devices due to sufficient transfer dynamics, The β-TiOPc was applied in perovskite solar cells without clopping to afford an impressive PCE of 5.05% under AM 1.5G illumination at the thickness of 40 nm which is competitive with spiro-OMeTAD at the same condition. The present work suggests a guideline for optimizing the photovoltaic properties ofperovskite solar cells using the TiOPc as the HTM.

Tetraphenylethene-based hole transporting material for highly efficient and stable perovskite solar cells

2,2,7,7-Tetrakis-(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene(Spiro-OMeTAD)has been identified as the most widely used and effective hole transporting material(HTM)in perovskite solar cells(PSCs).However,the complicated multistep synthesis and low intrinsic hole mobility of Spiro-OMe TAD limit its commercialized application.Therefore,developing highly efficient HTMs with less synthetic steps becomes increasingly important.Moreover,understanding hot carriers transfer dynamics at the interface of perovskite layer and hole transport layer is crucial for further enhancing PSCs performance towards Shockley-Queisser limit,which still lacks full investigation to date.Herein,a new HTM based on tetraphenylethene(WP1)was successfully synthesized by a simple one-step reaction process.It was found that WP1-based HTM exhibits more matched energy level,higher hole mobility and conductivity than those of the control Spiro-OMe TAD.The femtosecond transient absorption results reveal that the transfer rate of hot holes in perovskite/WP1 sample is four times higher than that of perovskite/Spiro-OMeTAD,thereby helping enhance the device performance.Consequently,the efficiency of PSCs is enhanced to 24.04%(WP1)from 22.85%(Spiro-OMeTAD).Moreover,the un-encapsulated device prepared with WP1 exhibits better long-term stability,retaining 87%of its initial PCE value after storing for 72 days under air environment,while the reference device shows76%of its initial value.This work indicates that simple tetraphenylethene-based organic small molecule could be a very promising HTM candidate for highly efficient PSCs,and gives some significant insights for understanding intrinsic hot carriers transfer dynamics in device.

Novel hole transport layer of nickel oxide composite with carbon for high-performance perovskite solar cells

A depth behavioral understanding for each layer in perovskite solar cells （PSCs） and their inter[acial interactions as a whole has been emerged for further enhancement in power conversion efficiency （PCE）. Herein, NiO@Carbon was not only simulated as a hole transport layer but also as a counter electrode at the same time in the planar heterojunction based PSCs with the program wxAMPS （analysis of microelectronic and photonic structures）-lD. Simulation results revealed a high dependence of PCE on the effect of band offset between hole transport material （HTM） and perovskite layers. Meanwhile, the valence band offset （AEv） of NiO-HTM was optimized to be -0.1 to -0.3 eV lower than that of the perovskite layer. Additionally, a barrier cliff was identified to significantly influence the hole extraction at the HTM/absorber interface. Conversely, the AEv between the active material and NiO@Carbon-HTM was derived to be -0.15 to 0.15 eV with an enhanced efficiency from 15% to 16%.

Recent Progresses on Dopant-Free Organic Hole Transport Materials toward Efficient and Stable Perovskite Solar Cells

As the third generation new battery,the power conversion efficiency(PCE)of metal halide perovskite solar cells(PsCs)has increased from 3.8%in 2009 to 25.8%currently certified,which fully shows that they have great research value and development prospect.As one of the main components of high-efficiency PSCs,hole transport materials(HTMs)play an important role in extracting and transporting holes and inhibiting charge recombination.However,commonly used HTMs require doping,and the hygroscopicity and corrosiveness of the dopants will destroy the stability of PsCs and hinder their commercialization.Therefore,it is of great significance to develop dopant-free HTMs.

Effective Improvement of the Photovoltaic Performance of Carbon-Based Perovskite Solar Cells by Additional Solvents

A solvent-assisted methodology has been developed to synthesize CH_3NH_3 PbI_3perovskite absorber layers.It involved the use of a mixed solvent of CH_3NH_3 I,PbI_2,c-butyrolactone,and dimethyl sulfoxide(DMSO) followed by the addition of chlorobenzene(CB).The method produced ultra-flat and dense perovskite capping layers atop mesoporous TiO_2 films,enabling a remarkable improvement in the performance of free hole transport material(HTM) carbon electrode-based perovskite solar cells(PSCs).Toluene(TO) was also studied as an additional solvent for comparison.At the annealing temperature of 100 °C,the fabricated HTM-free PSCs based on drop-casting CB demonstrated power conversion efficiency(PCE) of 9.73 %,which is 36 and 71 % higher than those fabricated from the perovskite films using TO or without adding an extra solvent,respectively.The interaction between the PbI_2–DMSO–CH_3NH_3I intermediate phase and the additional solvent was discussed.Furthermore,the influence of the annealing temperature on the absorber film formation,morphology,and crystalline structure was investigated and correlated with the photovoltaic performance.Highly efficient,simple,and stable HTM-free solar cells with a PCE of 11.44 % were prepared utilizing the optimum perovskite absorbers annealed at 120 °C.

Stable perovskite solar cells with efficiency of 22.6%via quinoxaline-based polymeric hole transport material

Poor stability of spiro-OMe TAD hole transport materials(HTMs)with dopant is a major obstacle for the commercialization of perovskite solar cells(pero-SCs).Herein,we demonstrate a series of quinoxaline-based D-A copolymers PBQ5,PBQ6 and PBQ10 as the dopant-free polymer HTMs for high performance pero-SCs.The D-A copolymers are composed of fluorothienyl benzodithiophene(BDTT)as D-unit,difluoroquinoxaline(DFQ)with different side chains as A-unit,and thiophene asπ-bridge,where the side chains on the DFQ unit are bi-alkyl for PBQ5,bi-alkyl-fluorothienyl for PBQ6,and alkoxyl for PBQ10.All the three copolymers are adopted as the dopant-free HTM in the pero-SCs.The planar n-i-p structured pero-SCs based on(FAPb I_(3))_(0.98)(MAPb Br_(3))_(0.02)with PBQ6 HTM demonstrated the high power conversion efficiency(PCE)of 22.6%with Vocof1.13 V and FF of 80.8%,which is benefitted from the suitable energy level and high hole mobility of PBQ6.The PCE of 22.6%is the highest efficiency reported in the n-i-p structured pero-SCs based on dopant-free D-A copolymer HTM.In addition,the peroSCs show significantly enhanced ambient,thermal and light-soaking stability compared with the devices with traditional spiroOMe TAD HTM.

Boosting efficiency and stability of perovskite solar cells with nickel phthalocyanine as a low-cost hole transporting layer material

The efficiency of perovskite solar cells（PSCs） has increased from around 4% to over 22% following a few years of intensive investigation. For most PSCs, organic materials such as 2,2＇,7,7＇-tetrakis（N,Npdimethoxyphenylamino）-9,9＇-spirobifluorene（spiro-OMeTAD） are used as the hole transporting materials（HTMs）, which are thermally and chemically unstable and also expensive. Here, we explored nickel phthalocyanine（NiPc） as a stable and cost-effective HTM to replace the conventionally used spiroOMeTAD. Because of its high carrier mobility and proper band alignments, we achieved a PCE of 12.1% on NiPc based planar device with short-circuit current density（Jsc） of 17.64 mAcm（-2）, open circuit voltage（Voc） of 0.94 V, and fill factor（FF） of 73%, outperforming the planar device based on copper phthalocyanine（CuPc） that is an outstanding representative of metal phthalocyanines（MPcs） reported. Moreover,the device with NiPc shows much improved stability compared to that based on the conventional spiroOMeTAD as a result of NiPc＇s high stability. Photoluminescence（PL） and Impedance spectroscopy analysis results show that thermally deposited NiPc has good hole-extraction ability. Our results suggest that NiPc is a promising HTM for the large area, low cost and stable PSCs.