Soluble butyl substituted copper phthalocyanine as alternative hole-transporting material for solution processed perovskite solar cells

Newfangled progressions in the charge transport layers impacting the stability and efficiency of perovskite solar cells

Organic-inorganic lead halide perovskite solar cells have rapidly emerged as a newfangled material for solar energy harnessing. Perovskite solar cells have succeeded in gaining a power conversion efficiency of 25% in the last year, further enhancement in the efficiency is anticipated due to advanced engineering of the different components making up the complete cell architecture with enhanced performance, stability and efficiency. Significant components of perovskite solar cell configurational architecture are the electron transport layer, active perovskite absorber layer, hole transport layer and counter electrode. Considering the profound role of transport layers in charge mobility, current review has particularly elucidated the advancements in the charge transport layers. The time duration of the review is from 2010 to 2021. However, the special focus has been laid on the recent articles. The influence of different organic and inorganic materials used for development of transport layers influencing the cell performance have been summarized. Materials used for transport layers have been modified by utilization of myriad of engineered substances through doping and surface functionalization strategies but every method have been marked by posing serious challenges towards the stability and efficiency of the cell and thus, hindering its commercialization. The review also provides an elucidation of the mechanical challenges and abatement strategies. These strategies are associated with the charge transport layers for enhancement of cell functionality.

Sustainable Approaches to the Synthesis of Metallophthalocyanines in Solution

This work aims to investigate more sustainable reaction conditions for the synthesis of metallophthalocyanines. Anisole, glycerol and their mixtures have been investigated as reaction media for the tetramerization of phthalonitriles. Acetates of three divalent first-transition metal cations, Co(II), Cu(II) and Zn(II), were used and several bases were tested, depending on the chosen substrates and reaction conditions, with a view to making the whole process more sustainable while ensuring its scalability. Unsubstituted phthalocyanines were synthesized to analyze the behavior of the different metal ions in terms of reactivity in the new reaction media, resulting in a general Cu > Co > Zn trend, while the nonpolar tetra-tert-butyl substitution was investigated to evaluate the synthesis of soluble derivatives in the new conditions. Furthermore, the potassium hydroxide (KOH)-aided statistical synthesis of the unsymmetrical 9(10), 16(17), 23(24)-tri-tert-butyl-2-iodophthalocyaninato zinc(II), starting from 4-tert-butylphthalonitrile and 4-iodophthalonitrile in a glycerol/anisole mixture, proceeded with a satisfactory 26% yield. Our results provide insights into the investigation of new reaction environments and the understanding of their strengths and weaknesses, with a view to further increasing the sustainability of the synthesis of metallomacrocycles with high added value while lowering their production cost.

Phthalocyanines and porphyrinoid analogues as hole- and electron-transporting materials for perovskite solar cells

Organic-inorganic lead halide perovskite absorbers in combination with electron and hole transporting selective contacts result in power conversion efficiencies of over 23% under AM 1.5 sun conditions. The advantage of perovskite solar cells is their simple fabrication through solution-processing methods either in n-i-p or p-i-n configurations. Using TiO₂ or SnO₂ as an electron transporting layer, a compositionally engineered perovskite as an absorber layer, and Spiro-OMeTAD as a HTM, several groups have reported over 20% efficiency. Though perovskite solar cells reached comparable efficiency to that of crystalline silicon ones, their stability remains a bottleneck for commercialization partly due to the use of doped Spiro-OMeTAD. Several organic and inorganic hole transporting materials have been explored to increase the stability and power conversion efficiency of perovskite solar cells. IIn this review, we analyse the stability and efficiency of perovskite solar cells incorporating phthalocyanine and porphyrin macrocycles as hole- and electron transporting materials. The π-π stacking orientation of these macrocycles on the perovskite surface is important in facilitating a vertical charge transport, resulting in high power conversion efficiency.

Soluble hexamethyl-substituted subphthalocyanine as a dopant-free hole transport material for planar perovskite solar cells

Boron subphthalocyanine (SubPc) has special physical and chemical properties, originating from its non-centrosymmetric, near-planar taper structure and large conjugated system; it can act as an alternative to the small molecule hole-transporting material 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene in perovskite solar cells (PSCs). To achieve a higher solubility in common organic solvents and a more suitable highest occupied molecular orbital energy level that aligns with the valence band of the perovskite material, a SubPc molecule with a hexamethyl substitution at its peripheral position (Me6-SubPc) was successfully designed and synthesized in a one-step method. Completely solution processed PSCs were fabricated with only a small hysteresis, a power conversion efficiency of 6.96% and Voc of 0.986 V.

Oxasmaragdyrins as New and Efficient Hole-Transporting Materials for High-Performance Perovskite Solar Cells

The high performance of the perovskite solar cells (PSCs) cannot be achieved without a layer of efficient hole-transporting materials (HTMs) to retard the charge recombination and transport the photogenerated hole to the counterelectrode. Herein, we report the use of boryl oxasmaragdyrins (SM01, SM09, and SM13), a family of aromatic core-modified expanded porphyrins, as efficient hole-transporting materials (HTMs) for perovskite solar cells (PSCs). These oxasmaragdyrins demonstrated complementary absorption spectra in the low-energy region, good redox reversibility, good thermal stability, suitable energy levels with CH₃NH₃PbI₃ perovskite, and high hole mobility. A remarkable power conversion efficiency of 16.5% (V_oc = 1.09 V, J_sc = 20.9 mA cm^-2, fill factor (FF) = 72%) is achieved using SM09 on the optimized PSCs device employing a planar structure, which is close to that of the state-of-the-art hole-transporting materials (HTMs), spiro-OMeTAD of 18.2% (V_oc = 1.07 V, J_sc = 22.9 mA cm^-2, FF = 74%). In contrast, a poor photovoltaic performance of PSCs using SM01 is observed due to the interactions of terminal carboxylic acid functional group with CH₃NH₃PbI₃.

Interfacial Engineering of Perovskite Solar Cells by Employing a Hydrophobic Copper Phthalocyanine Derivative as Hole-Transporting Material with Improved Performance and Stability

In high-performance perovskite solar cells (PSCs), hole-transporting materials (HTMs) play an important role in extracting and transporting the photo-generated holes from the perovskite absorber to the cathode, thus reducing unwanted recombination losses and enhancing the photovoltaic performance. Herein, solution-processable tetra-4-(bis(4-tert-butylphenyl)amino)phenoxy-substituted copper phthalocyanine (CuPc-OTPAtBu) was synthesized and explored as a HTM in PSCs. The optical, electrochemical, and thermal properties were fully characterized for this organic metal complex. The photovoltaic performance of PSCs employing this CuPc derivative as a HTM was further investigated, in combination with a mixed-ion perovskite as a light absorber and a low-cost vacuum-free carbon as cathode. The optimized devices [doped with 6 % (w/w) tetrafluoro-tetracyano-quinodimethane (F4TCNQ)] showed a decent power conversion efficiency of 15.0 %, with an open-circuit voltage of 1.01 V, a short-circuit current density of 21.9 mA cm^-2 , and a fill factor of 0.68. Notably, the PSC devices studied also exhibited excellent long-term durability under ambient condition for 720 h, mainly owing to the introduction of the hydrophobic HTM interlayer, which prevents moisture penetration into the perovskite film. The present work emphasizes that solution-processable CuPc holds a great promise as a class of alternative HTMs that can be further explored for efficient and stable PSCs in the future.

Low cost and solution-processable zinc phthalocyanine as alternative hole transport material for perovskite solar cells

A solution-processable and dopant-free ZnPc(tBu)₄was used a potential cost-effective substitute for the expensive HTMs containing multifold dopants used in the current PSCs.

Diketopyrrolopyrrole or benzodithiophene-arylamine small-molecule hole transporting materials for stable perovskite solar cells

Two new hole transporting materials with diketopyrrolopyrrole or benzodithiophene moieties were developed for stable perovskite solar cells.