Design and synthesis of soluble dibenzosuberane-substituted fullerene derivatives for bulk-heterojunction polymer solar cells

Two new dibenzosuberane-substituted fullerene derivatives, dibenzosuberane-C₆₀ mono-adduct (DBSCMA) and bis-adduct (DBSCBA) were synthesized using a classical cyclopropanation reaction via a tosylhydrazone route for application as acceptor materials in polymer solar cells (PSCs). DBSCBA shows good solubility in common organic solvents and both derivatives were characterized by ¹HNMR, ¹³C NMR, MALD-TOF, elemental analysis and UV–vis absorption measurements. The shift of fullerene energy levels induced by the dibenzosuberane substitution was investigated by using theoretical simulations and ultraviolet photoelectron spectroscopy. Bulk-heterojunction PSCs based on poly (3-hexylthiophene) (P3HT) and dibenzosuberane-C₆₀ derivatives were fabricated and optimized by adjusting the donor/acceptor ratio and using thermal annealing and solvent additive. The morphologies of the active layers processed under different conditions were also examined by atomic force microscopy. When tested under an illumination of AM 1.5 G at 100 mW/cm², the highest power conversion efficiency of the devices using DBSCBA is 3.70% which is superior to that of conventional P3HT:PCBM devices.     

Reversible photoinduced bi-state polymer solar cells based on fullerene derivatives with azobenzene groups

[6,6]-Phenyl-C61-butyric acid-4′-hydroxyl-azobenzene ester (PCBAb) was synthesized and used as the acceptor in the fabrication of reversible UV–VIS response bi-state polymer solar cells (PSCs) based on the photoinduced cis–trans isomerization of PCBAb. The device can be switched between “active” and “sleep” by the irradiation of UV and visible light, respectively. The active device has a PCE of 2.0%. With UV irradiation, the device goes to “sleep” with a lowered PCE (0.4%), and simultaneously decreased J_sc, V_oc and FF, while after visible light treatment, the device is made “active” again. The mechanism of the bi-state process involves the different electron mobilities of the isomers.     

Mixed C60/C70 based fullerene acceptors in polymer bulk-heterojunction solar cells

Different mixtures of identically substituted C60 and C70 based fullerens have been used as acceptors in three polymer:fullerene systems that strongly express various performance limiting aspects of bulk heterojunction solar cells. Results are correlated with, and discussed in terms of e.g. morphology, charge separation, and charge transport. In these systems, there appears to be no relevant differences in either mobility or energy level positions between the identically substituted C60 and C70 based fullerenes tested. Examples of how fullerene mixtures influence the nano-morphology of the active layer are given. An upper limit to the open circuit voltage that can be obtained with fullerenes is also suggested.     

Efficiency enhancement of polymer solar cells by applying an alcohol-soluble fullerene aminoethanol derivative as a cathode buffer layer

Cathode buffer layer (CBL) introduced between the active layer and cathode is crucial for selectively transporting electrons and blocking holes for polymer solar cells (PSCs). Calcium (Ca) is the most commonly used CBL in conventional-structure bulk heterojunction (BHJ) PSC devices, but is prone to oxidation due to its high reactivity, inhibiting its practical applications. Herein, we applied an alcohol-soluble fullerene aminoethanol derivative (C₆₀-ETA) as an efficient CBL surpassing Ca in conventional-structure BHJ-PSC devices, leading to obvious efficiency enhancement with the best power conversion efficiency (PCE) reaching 9.66%. C₆₀-ETA CBL was applied in PSC devices based on three different photoactive layer systems, including poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate]:[6,6]-phenyl C₇₁-butyric acid methyl ester (PTB7-Th:PC₇₁BM), polythieno[3,4-b]thiophene-co-benzodithiophene (PTB7):PC₇₁BM and poly(4,8-bis-alkyloxybenzo(l,2-b:4,5-b′)dithiophene-2,6-diylalt-(alkylthieno(3,4-b)thiophene-2-carboxylate)-2,6-diyl) (PBDTTT-C):PC₇₁BM, affording the best PCE of 9.66%, 8.51% and 7.19%, respectively, which are all higher than those of the corresponding devices based on the commonly used Ca CBL. The mechanism of efficiency enhancement of C₆₀-ETA CBL relative to Ca is studied, revealing that C₆₀-ETA CBL may induce improvements on both the interfacial contact between the active layer/cathode and electron transport, facilitating electron extraction by the Al cathode, and consequently leading to the increase of short-circuit current density (J_sc), which contributes primarily to the PCE improvement.     

Influences of charge of conjugated polymer electrolytes cathode interlayer for bulk-heterojunction polymer solar cells

Two fluorene-based conjugated polymer electrolyte (CPE) poly[(9,9-bis(6′-(N,N,N-trimethylammonium)hexyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFNBr) and poly[9,9-bis(4′-sulfonatobutyl)fluorene-alt-2,7-(9,9-dioctylfluorene)] sodium salt (PFSO₃Na), bearing amine groups and anionic sulfonate groups on side chains respectively, are synthesized and applied as cathode interlayer in polymer solar cells. Both of the hydrophilic CPEs can well modify the interfacial properties and allow ohomic contact between the activelayer and cathode. The opposite charges exert great influence on the effective work function of cathode and interfacial interaction through the orientation of the interfacial dipole at the active layer/metal electrode interface, subsequently influence the resulting device performance. Compared with the cationic PFNBr, PFSO₃Na with anionic sulfonate groups can dramatically reduce the work function of Al by accumulation of the polar groups at the PFSO₃Na/Al interface to induce more favorable the interfacial dipole. The better energy alignment for electron extraction and transportation at active layer/Al interface is confirmed by a significant enhancement of V_OC. The better wettability and morphology of PFSO₃Na on the active layer and the more effective motion of sodium counterion further modify the barrier to facilitate electron extraction and transportation. Moreover, 14% and 22% performance enhancement can also be achieved respectively, when PFNBr and PFSO₃Na are used as interlayers for low bandgap poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT)-based solar cells.     

Utilizing intermixing of conjugated polymer and fullerene from sequential solution processing for efficient polymer solar cells

Efficient polymer solar cells based on poly[2, 6-(4, 4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-fluorobenzothiadiazole)] (PCPDTFBT) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) are successfully fabricated by a sequential processing (SqP). With appropriate orthogonal solvent and thermal treatment, the SqP film can form an inter-diffused layer, and the SqP devices show efficient photovoltaic performance in both conventional and inverted layouts. The SqP inverted device was firstly demonstrated and the highest power conversion efficiency (PCE) of 5.84% with the enhanced J_sc of 16.4 mA cm⁻² was able to be achieved with the high internal quantum efficiency (IQE). Photoluminescence quenching shows the SqP films can provide efficient exciton quenching. X-ray photoemission spectroscopy (XPS) and ellipsometry analysis shows a polymer-rich surface in SqP films after thermal annealing. The charge mobilities in the SqP films were significantly enhanced as measured by space-charge-limited-current (SCLC) method. All these contribute to the improved photovoltaic performance in the inverted SqP device. We believe that these results inspire a new way of forming the active layer with controllable morphology, efficient charge separation and collection in polymer solar cells.     

Performance improvement of conventional and inverted polymer solar cells with hydrophobic fluoropolymer as nonvolatile processing additive

The morphology of the photoactive layer critically affects the performance of the bulk heterojunction polymer solar cells (PSCs). To control the morphology, we introduced a hydrophobic fluoropolymer polyvinylidene fluoride (PVDF) as nonvolatile additive into the P3HT:PCBM active layer. The effect of PVDF on the surface and the bulk morphology were investigated by atomic force microscope and transmission electron microscopy, respectively. Through the repulsive interactions between the hydrophilic PCBM and the hydrophobic PVDF, much more uniform phase separation with good P3HT crystallinity is formed within the active layer, resulting enhanced light harvesting and improved photovoltaic performance in conventional devices. The PCE of the conventional device can improve from 2.40% to 3.07% with PVDF additive. The PVDF distribution within the active layer was investigated by secondary ion mass spectroscopy, confirming a bottom distribution of PVDF. Therefore, inverted device structure was designed, and the PCE can improve from 2.81% to 3.45% with PVDF additive. Our findings suggest that PVDF is a promising nonvolatile processing additive for high performance polymer solar cells.     

Amphiphilic fullerene derivative as effective interfacial layer for inverted polymer solar cells

Amphiphilic fullerene derivative with poly(ethylene glycol) chain (C60-PEG) was applied as effective interfacial layer to improve the performance of inverted polymer solar cells. C60-PEG could not only be used as cathode buffer layer alone by replacing ZnO, but also be used as a self-assembled monolayer to modify ZnO. C60-PEG can tune energy level alignment and improve the interfacial compatibility between active layer and ITO or ZnO. Moreover, due to the strong interaction between ZnO nanoparticles and PEG chain, C60-PEG can passivate the surface defects and traps of ZnO, and facilitate the charge selective and dissociation. Consequently, inverted polymer solar cells based on thieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]- phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) present a PCE of 6.6% by incorporating C60-PEG into as cathode buffer layer. Moreover, an improved PCE of 7.4% with good long-term stability in air were further achieved by using C60-PEG/ZnO interlayer. In this work, C60-PEG could be prepared by solution process at room temperature without additional annealing, which shows the potential in large-scale printed polymer solar cells.     

Correlation between polymer molecular weight and optimal fullerene content in efficient polymer solar cells

In this contribution, a donor-acceptor (D-A) copolymer PTP8, consisting of alternating benzodithiophene and thienopyrroledione with conjugated side-chains on both donor and acceptor units, was sucessfully prepared. We further investigated the effect of polymer molecular weight on polymer physicochemical properties, solar cell device performance, polymer-PCBM blend morphology, and, most importantly, polymer/PCBM blend ratio. We found that increasing the molecular weight of the donor polymer can both effectively improve the device performance and simultaneously stabilize solar cell efficiency over a wide range of polymer/PCBM blend ratios (from 1:0.5 to 1:1.0), which may lead to more thermally stable and cost-effective devices. Through intensive morphological investigation, we propose a sound morphological evolution for PTP8/PCBM blends with different molecular weights at low fullerene content.     

Wide range thickness effect of hole-collecting buffer layers for polymer:fullerene solar cells

Here we report the wide range thickness effect of hole-collecting buffer layers (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)) on the performance of polymer:fullerene solar cells with blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM). The thickness of the PEDOT:PSS layers was controlled from 3 nm to 625 nm, followed by characterizations such as optical transmittance, electrical resistances in the in-plane and out-of-plane directions, work functions, contact angles, and device performances. Results showed that the optical transmittance was gradually decreased with the PEDOT:PSS thickness but a maximum value was measured for other properties in the thickness range of 10–30 nm. The device performance was noticeably improved with only 3 nm-thick PEDOT:PSS layer, while it was almost similar in the thickness range of 30–225 nm in the presence of gradual decrease in the surface roughness. The similar device performance between 30 nm and 225 nm has been assigned to the compensation effect between the reduced electrical resistance (increased conductivity) and the decreased optical transmittance as the thickness of the PEDOT:PSS layer increased.     

Dual structure modifications to realize efficient polymer solar cells with low fullerene content

The molecular structures of both donor polymer and acceptor fullerene were adjusted and their effect on the donor/acceptor blend ratio in the polymer solar cells was investigated. We found that increasing the side-chain rigidity of the donor polymer or bulkiness of the fullerene can both effectively reduce the fullerene intercalation into polymer side chains, realizing efficient electron transport at higher polymer/fullerene ratio. Especially, by using a bulkier fullerene molecule, all the three adopted polymers exhibit remarkably stable device performance over a wide range of blend ratio (from 1:0.5 to 1:1.0), which can't be achieved by using conventional PC₆₁BM. Moreover, using bulky PC₆₁BAd together with a high side-chain density polymer PTP8, the optimal device performance could be obtained at a surprisingly low blend ratio of 1:0.6, which may lead to more thermally stable and cost-effective devices.     

Light-trapping in polymer solar cells by processing with nanostructured diatomaceous earth

We demonstrate the use of fossilized diatoms (diatomaceous earth) as light traps in regioregular poly(3-hexylthiophene) (P3HT) and fullerene derivative [6,6]-phenyl-C₆₀-butyric acid methyl ester (PCBM) solar cells. Diatoms, the most common type of phytoplankton found in nature, are optimized for light absorption through millions of years of adaptive evolution. They are also an earth-abundant source of silica that can be incorporated into polymer solar cells without the need for complicated processing. Here we establish protocols dispersing the diatomaceous earth throughout the P3HT:PCBM active layer with characterization by optical and current-voltage measurements. We show that through the addition of diatomaceous earth, we can achieve the same power conversion efficiencies as standard thickness cells while using 36% thinner active layers. We find that adding the diatomaceous earth acts as a scattering center and textures the silver back contact, contributing to increases in the optical path length within devices. Results from this study open up pathways for incorporating hierarchical materials from nature into energy conversion devices.     

Optical,electrical and mechanical properties of indium tin oxide on polyethylene terephthalate substrates: Application in bulk-heterojunction polymer solar cells

We investigated optical, electrical and mechanical properties of indium tin oxide (ITO) on flexible polyethylene terephthalate (PET) substrate, considering bulk-heterojunction (BHJ) polymer solar cells applications. Encapsulation of flexible solar cells with the architecture PET/ITO/PEDOT:PSS/P3HT:PCBM (or P3HT:PCBM:AZ-NDI-4)/Al was done by direct brush-painting with nail enamel. Active cell layer blends of [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) with regioregular or regiorandom poly(3-hexylthiophene-2,5-diyl) (P3HT) were applied. Additionally for this role the mixture of regioregular P3HT:PCBM with naphthalene diimide–imine with four thiophene rings AZ-NDI-4 was tested. Obtained photovoltaic (PV) and optical (UV–vis) results of the flexible polymer solar cells were compared with the same architecture of devices on the glass/ITO substrate.     

Efficiency improvement of polymer solar cells by iodine doping

A series of poly(3-hexylthiophene) (P3HT)/(6,6)-phenyl C₆₀ butyric acid methyl ester (PCBM) bulk hetero-junction polymer solar cells were fabricated with different iodine (I₂) doping concentrations. The short circuit current density (J_sc) was increased to 8.7 mA/cm² from 4 mA/cm², meanwhile the open circuit voltage (V_oc) was decreased to 0.52 V from 0.63 V when the iodine doping concentration is 5%. The optimized power conversion efficiency of polymer solar cells (PSCs) with iodine doping is about 1.51%, which should be attributed to the better charge carrier transport and collection, and the more photon harvesting due to the red shift of absorption peaks and the widened absorption range to the longer wavelength. The morphology and phase separation of polymer thin films were measured by atomic force microscopy (AFM). The phase separation of P3HT and PCBM has been distinctly increased, which is beneficial to the exciton dissociation. The photocurrent density of PSCs with iodine doping was increased compared with the PSCs without iodine doping under the same effective voltage.     

Dramatically enhanced performances and ideally controlled nano-morphology via co-solvent processing in low bandgap polymer solar cells

The device performance of photovoltaics with a polymer:fullerene bulk heterojunction (BHJ) structure, consisting of DT-PDPP2T-TT donor polymer and poly(3-hexylthiophene):[6,6]phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) acceptor compound, was investigated as a function of co-solvent composition. An enhancement of the photocurrent density and fill factor is observed in diodes made by spin-coating with chloroform mixed with ortho-dichlorobenzene, which allows a significantly higher device efficiency of 5.55% compared to diodes made from neat chloroform (efficiency = 3.61%). To clarify the role of the co-solvent, we investigated the nanoscale morphology with AFM, TEM and 2D-GIWAXS techniques and also the free-charge carrier mobility via space-charge limited current theory. We obtained the result that, under such supersaturated conditions, co-solvents induce increased polymer crystalline aggregation into a 3D phase structure and boost charge-carrier transport characteristics. This provides a rational basis for the development of ideally-controlled BHJ films that yield efficient DT-PDPP2T-TT:PCBM solar cells. Therefore, carefully selecting solvent mixtures provides an approach toward efficient low bandgap polymer solar cells.     

Miscibility in binary blends of non-peripheral alkylphthalocyanines and their application for bulk-heterojunction solar cells

The fabrication of small-molecule bulk-heterojunction solar cells utilizing a mixed donor material composed of two types of soluble phthalocyanine derivatives with different substituent length has been studied. The power conversion efficiency (PCE) and short-circuit current density (J_sc) of the solar cells fabricated using the mixed donor material with an optimized mixture ratio reached 3.8% and 9.2 mA/cm², respectively, which were superior to those of organic solar cells utilizing each type of phthalocyanine derivative as a single donor material. The improvement of PCE and J_sc has been discussed from the viewpoints of the miscibility and carrier transport properties of the mixed donor material.     

Fluorine functionalized asymmetric indo [2,3-b]quinoxaline framework based D-A copolymer for fullerene polymer solar cells

Innovating molecular structure of copolymer donor materials is still one of the prominent approach to obtain high-performance polymer solar cells (PSCs). In this paper, two novel wide bandgap (WBG) copolymers, namely PBDTTS-IQ and PBDTTS-DFIQ, based on asymmetric planar aromatic core indo [( Li et al., 2012; Wang et al., 2020) 2,32,3-b]quinoxaline (IQ) as acceptor unit through tuning side chains with fluorine (F) atom engineering and exemplary alkylthio-thienyl substituted benzodithiophene (BDTTS) donor group, are synthesized and finally employed as the photovoltaic donor materials for fullerene polymer solar cells (PSCs). After blending with PC₇₁BM acceptor, the PBDTTS-DFIQ:PC₇₁BM blend film presented better efficient exciton dissociation and charge extraction, more balanced electron/hole mobility (μ_h/μ_e), and nice morphology in comparison with PBDTTS-IQ:PC₇₁BM blend film. Encouragingly, the PBDTTS-DFIQ:PC₇₁BM based PSCs exhibits a higher power conversion efficiency (PCE) of 7.4% than that of the device based on the PBDTTS-IQ:PC₇₁BM blend with a PCE of 4.96%, which thanks to an enhancement of open-circuit voltage (V_oc) of 0.84 V, short current density (J_sc) of 13.26 mA cm⁻² and fill factor (FF) of 66.00% simultaneously. These results demonstrate that this asymmetric IQ framework is a wonderful acceptor moiety to build light-harvesting copolymers for highly efficient PSCs.     

End-chain effects of non-fullerene acceptors on polymer solar cells

Four acceptor1-acceptor2-donor-acceptor2-acceptor1 (A1-A2-D-A2-A1) structural electron acceptors with different end-chains were designed and synthesized which all possessed indacenodithiophene (IDT) core, benzothiadiazole (BT) bridge as acceptor2, and rhodanine (R) end groups as acceptor1. The non-fullerene acceptor attached with ethyl group is called IDT-BT-R2 and used as control compound. And the other three of them are attached with methoxymethyl, trifluoroethyl and 1-piperidino groups generating IDT-BT-RO, IDT-BT-RF3 and IDT-BT-RN, respectively. The influence of end-chains on their optoelectronic properties were compared between four non-fullerene acceptors. Compared with IDT-BT-R2, the molecule IDT-BT-RF3 show red-shifted light absorption and lower LUMO level because of the electron withdrawing property of fluorine atoms. OSCs based on IDT-BT-RF3 display more efficient charge separation and lower degree of monomolecular recombination, allowing OSCs to show higher short-circuit current (J_sc) than the system of IDT-BT-R2. OSCs based on IDT-BT-RO also show more efficient charge separation and less monomolecular recombination. Due to the elevated LUMO level of the acceptor IDT-BT-RN, organic solar cells (OSCs) utilizing this material as acceptor display high open-circuit voltage (V_oc) of 1.10 eV and low energy loss of 0.49 eV when maintaining a relatively high power conversion efficiency (PCE) of 7.09%. We demonstrated that the end-chain engineering could finely tune the light absorption properties and energy levels of novel non-fullerene acceptors and eventually improved OSCs performance can be harvested.     

Transient photovoltage and dark current analysis on enhanced open-circuit voltage of polymer solar cells with hole blocking TiO2 nanoparticle interfacial layer

The open-circuit voltage of bulk heterojunction polymer solar cells utilizing 1,8-diiodooctane (DIO) as a processing additive was greatly improved by using an organic layer coated TiO₂ nanoparticle interfacial layer inserted between the active layer and the Al electrode. The transient photovoltage measurement revealed that there was significant non-geminate recombination at the DIO-processed active layer/Al electrode interface. Reduced open-circuit voltage (V_OC) of the photovoltaic devices and high water contact angle of the DIO-processed active layer showed that the DIO-processed active layer has an undesirable surface composition for the electron collection. The organic layer coated TiO₂ nanoparticle interfacial layer effectively prevented the non-geminate recombination at the active layer/Al interface. As a result, we were able to significantly improve the V_OC and power conversion efficiency from 0.46 V and 2.13% to 0.62 V and 3.95%, respectively.     

Role of fullerene electron transport layer on the morphology and optoelectronic properties of perovskite solar cells

High performance, hysteresis-free, low temperature n-i-p perovskite solar cells are successfully fabricated by solution processing using fullerene electron transport layer (ETL). PC₇₁BM fullerene, with broader absorption spectrum and lower HOMO level, when incorporated in the perovskite solar cell yielded average power conversion efficiency (PCE) of 13.9%. This is the highest reported PCE in n-i-p perovskite solar cells with PC₇₁BM ETL. The devices exhibited negligible hysteresis and high open-circuit voltage (V_oc). On the contrary, devices with PC₆₁BM, a common fullerene ETL in perovskite solar cell, exhibited large hysteresis and lower V_oc. The underlying mechanisms of superior performance of devices with PC₇₁BM ETL were found to be correlated with fullerene surface wettability and perovskite grain size. The influence of fullerene ETL on the perovskite grain growth and subsequent photovoltaic performance was investigated by contact angle measurement, morphological characterization of the surface topography and electrochemical impedance analysis.