Donor–acceptor–acceptor–donor small molecules for solution processed bulk heterojunction solar cells

We report on the optical and electrochemical characterization (experimental and theoretical) of two donor substituted benzothiadiazole with different cyano based acceptor π-linkers, tetracyanobutadiene (TCBD) SM1 and dicyanoquinomethane (DCNQ) SM2, and explore them as the donor component for solution processed bulk heterojunction organic solar cells, along with PC₇₁BM as the electron acceptor. The solution bulk heterojunction (BHJ) solar cells based on dichloromethane (DCM) processed active layer with SM1 and SM2 as donor and PC₇₁BM as acceptor achieve power conversion efficiency (PCE) of 2.76% and 3.61%, respectively. The solar cells based on these two small molecules exhibit good Voc, which is attributed to their deep HOMO energy level. The higher PCE of the device based on SM2 compared to SM1 is attributed to the its small bandgap, broader absorption profile and enhanced hole mobility. Additionally, the PCE of the SM2:PC₇₁BM based solar cells processed with 1-chloronaphthalene CN (3 v%)/DCM is further improved reaching upto 4.86%. This increase in PCE has been attributed to the improved nanoscale morphology and more balanced charge transport in the device, due to the solvent additive.     

Oligothiophene-based small molecules with 3,3′-difluoro-2,2′-bithiophene central unit for solution-processed organic solar cells

Two new oligothiophene-based small molecules, namely DRCN6T-F and DRCN8T-F, with 3,3′-difluoro-2,2′-bithiophene as the central building block and 2-(1,1-dicyanomethylene)-rhodanine as end groups, were designed and synthesized. Compared to their non-fluorinated counterparts DRCN6T and DRCN8T, DRCN6T-F and DRCN8T-F exhibit enhanced intermolecular interactions and lower HOMO energy levels. However, PCEs of 2.26% and 5.07% were obtained for DRCN6T-F and DRCN8T-F based optimized devices, respectively, lower than those of non-fluorinated molecules DRCN6T and DRCN8T. The relatively poor performance for the DRCN6T-F and DRCN8T-F were mainly caused by their low short-circuit current densities, due to their unfavorable morphologies and low charge carrier mobilities.     

Linear and propeller-like fluoro-isoindigo based donor–acceptor small molecules for organic solar cells

Two donor–acceptor type fluoro-isoindigo based small molecule semiconductors are synthesized and their optical, electrochemical, thermal, and charge transport properties are investigated. The two molecular chromophores differ by their architecture, linear (M1) vs propeller-like (M2). Both molecules present a broad absorption in the visible range and a low optical HOMO–LUMO gap (∼1.6 eV). AFM images of solution-processed thin films show that the trigonal molecule M2 forms highly oriented fibrils after a few seconds of solvent vapor annealing. The materials are evaluated as electron donor components in bulk heterojunction organic solar cells using PC₆₁BM as the electron acceptor. The devices based on the propeller-like molecule M2 exhibit a high open-circuit voltage (around 1.0 V) and a power conversion efficiency of 2.23%.     

Two new A-D-A type small molecule acceptors based on C2v-symmetric dithienocyclopentaspiro[fluorene-9,9′-xanthene] core for polymer solar cells

A C_2v-symmetric core, dithienocyclopentaspiro[fluorene-9,9′-xanthene], was used as the central block for the first time to design and synthesize A-D-A type small molecule acceptors for nonfullerene polymer solar cells (PSCs), and two new small molecule acceptors of TSFX-2F and TSFX-4F were synthesized based on the C_2v-symmetric core. The two TSFX-based acceptors show high thermal stability, strong absorption in the wavelength region of 550–750 nm and appropriate energy levels. The PSCs with the broad bandgap polymer J71 as donor and TSFX-2F as acceptor demonstrated power conversion efficiency (PCE) of 9.42% with open circuit voltage (V_oc) of 0.89 V, short circuit current density (J_sc) of 15.27 mA cm⁻² and fill factor (FF) of 69.30%, while the PSC based on J71:TSFX-4F shows a PCE of 8.47% with V_oc of 0.83 V, J_sc of 15.48 mA cm⁻² and FF of 66.16%. The higher V_oc of the PSC based on J71: TSFX-2F is benefitted from the up-shifted LUMO energy level of the TSFX-2F acceptor, and its higher FF can be ascribed to the higher and more balanced hole and electron mobilities of the J71: TSFX-2F active layer. This work demonstrates that the new C_2v-symmetric building block is a promising central D-unit for the design and synthesis of new structured norfullerene acceptors for high-performance PSCs.     

Efficient solution processed D1-A-D2-A-D1 small molecules bulk heterojunction solar cells based on alkoxy triphenylamine and benzo[1,2-b:4,5-b′]thiophene units

Two molecules denoted as VC96 and VC97 have been synthesized for efficient (η = 6.13% @ 100 mW/cm² sun-simulated light) small molecule solution processed organic solar cells. These molecules have been designed with the D₁-A-D₂-A-D₁ structure bearing different central donor unit, same benzothiadiazole (BT) as π-acceptor and end capping triphenylamine. Moreover, the optical and electrochemical properties (both experimental and theoretical) of these molecules have been systematically investigated. The solar cells prepared from VC96:PC₇₁BM and VC97:PC₇₁BM (1:2) processed from CF (chloroform) exhibit a PCE (power conversion efficiency) of η = 4.06% (J_sc = 8.36 mA/cm², V_oc = 0.90 V and FF = 0.54) and η = 3.12% (J_sc = 6.78 mA/cm², V_oc = 0.92 V and FF = 0.50), respectively. The higher PCE of the device with VC96 as compared to VC97 is demonstrated to be due to the higher hole mobility and broader IPCE spectra. The devices based on VC96:PC₇₁BM and VC97:PC₇₁BM processed with solvent additive (3 v% DIO, 1,8-diiodooctane) showed PCE of η = 5.44% and η = 4.72%, respectively. The PCE device of optimized VC96:PC₇₁BM processed with DIO/CF (thermal annealed) has been improved up to 6.13% (J_sc = 10.72 mA/cm², V_oc = 0.88 V and FF = 0.61). The device optimization results from the improvement of the balanced charge transport and better nanoscale morphology induced by the solvent additive plus the thermal annealing.     

D–A–Ar-type small molecules with enlarged π-system of phenanthrene at terminal for high-performance solution processed organic solar cells

Two novel D–A–Ar-type small molecules of TPA–DPP–P and TPA(DPP–P)₂ were synthesized and characterized, in which triphenylamine (TPA), diketopyrrolopyrrole (DPP) and phenanthrene (P) were used as the donor (D) core, acceptor (A) arm, and enlarged π-system of polycyclic arene (Ar) terminal. Their absorptive, electro-chemical, thermal, and photovoltaic properties were preliminary investigated. Significantly improved photophysical and photovoltaic performances were observed for both small molecules containing the planar P terminal in comparison with those for their parent D–A-type molecule of TPA–DPP. The highest power conversion efficiency (PCE) of 3.42% and a maximum short-circuit current density (J_sc) of 9.2 mA/cm² were obtained in the solution-processed TPA(DPP–P)₂-based solar cells using [6,6]-phenyl-C-71-butyric acid methyl ester (PC₇₁BM) as acceptor. The PCE and J_sc values are 8.76 and 4.97 times higher than those of the TPA–DPP-based cells, respectively. It indicates that appending the enlarged π-system of the planar P terminal and incorporating the DPP–P arm into D–A-type small molecule are efficient approaches to improve photophysical and photovoltaic performances for its resulting molecules.     

A strategy to enhance both VOC and JSC of A–D–A type small molecules based on diketopyrrolopyrrole for high efficient organic solar cells

Four different diketopyrrolopyrrole (DPP)-based small molecules (SMs) with A–D–A type structure were synthesized, where electron-donating unit (D) was systematically varied with different electron-donating power (thiophene vs. phenylene; thienothiophene vs. naphthalene) and different molecular planarity (bithiophene vs. thienothiophene; and biphenylene vs. naphthalene). The small molecules with weak donating unit (phenylene or naphthalene) have deeper HOMO energy levels than those with strong donating unit (thiophene or thienothiophene), and thus exhibit higher V_OC. When the fused aromatic ring (thienothiophene or naphthalene) with planar molecular structure is introduced in SMs, the SMs exhibit high hole mobility and thus afford high J_SC. As a result, the introduction of naphthalene (weak donating power and planar structure) enhances both V_OC and J_SC, resulting in a promising power conversion efficiency of 4.4%. This result provides a valuable guideline for rational design of conjugated small molecules for high performance organic solar cells.     

Investigation of donor–acceptor copolymer films and their blends with fullerene in the active layers of bulk heterojunction solar cells by Raman microspectroscopy

Thin films made of three low-band gap donor–acceptor copolymers (CDTF, CDTDP and CDTDOP) composed of 4,6-bis(3′-dodecylthiophen-2′-yl)thieno[3,4-c][1,2,5]thiadiazole-5′,5′-diyl as an electron-acceptor structural unit and various electron-donor structural units, such as 9,9-bis(2-ethylhexyl)fluorene-2,7-diyl, 2,5-didodecyl-1,4-phenylene and 2,5-didodecyloxy-1,4-phenylene, respectively, and thin films of their blends with various ratios of a soluble fullerene derivative [6,6]-phenyl C₆₁-butyric acid methyl ester ([60]PCBM) as an active layer for bulk heterojunction solar cells were studied by means of UV–vis absorption spectroscopy and Raman microspectroscopy. The molecules of CDTDP and CDTDOP possess the same main chains; they differ in the side-chain oxygen only, which changes the donor strength of the donor units. UV–vis and Raman studies allow us to show differences in the hindering of molecule planarization and aggregation in the blends. Absorption of the polymer films covered the whole visible spectral region and extended up to near infrared for CDTDOP. The absorption behavior of the CDTDP blend films qualitatively differed from the absorption behavior of the blend films of CDTF or CDTDOP. The Raman measurements were performed at two different laser excitation wavelengths (633 and 785 nm), which enabled the photoluminescence of both components in the Raman spectra to be distinguished. The Raman study was performed in different parts of the films, including the separated areas. It was proven that the separated areas in the blend films had higher contents of [60]PCBM than the rest of the films.     

A–D–A type organic donors employing coplanar heterocyclic cores for efficient small molecule organic solar cells

Two linear organic A–D–A molecules (DTPT and DTPTT) comprised of electron-donating (D) coplanar heteroacenes as core end-capping with electron-accepting (A) dicyanovinylene were investigated as electron donor materials in organic photovoltaic (OPV) applications. The photophysical and electrochemical properties of these two dyes were examined. The A–D–A configuration renders these two molecules to have intense and red-shifted absorption characteristics for better light-harvesting (higher photocurrent density), while retaining relatively low HOMO energy levels for keeping sufficiently high open circuit voltage (V_oc) in OPV. The optical constants and molecular orientation of thin films were acquired with variable-angle spectroscopic ellipsometry (VASE). Due to the anisotropic behavior observed in thin film, these two organic donors were firstly adopted to combine with electron acceptor C₆₀ in a vacuum-processed planar heterojunction (PHJ) solar cells. The optimized DTPT-based PHJ device yielded a PCE of 3.01%, whereas the PHJ device based on DTPTT, delivered an inferior PCE of 1.70%. The exciton diffusion length extracted from spectrum-response modeling of PHJ devices is ∼5 nm and ∼4 nm for DTPT and DTPTT, respectively. Replacement of C₆₀ with C₇₀ for a better spectral response in 400–500 nm, planar-mixed heterojunction (PMHJ) SMOSCs without a thin donor layer in between the active layer and MoO₃ was found to produce optimum device results. The optimized DTPTT-based device showed a PCE of 3.02%, while the shorter counterpart DTPT delivered a PCE up to 5.64%.     

Acetylene-bridged D–A–D type small molecule comprising pyrene and diketopyrrolopyrrole for high efficiency organic solar cells

To explore effects of acetylene-incorporation, acetylene-bridged D–A–D type small molecules ((HD/OD)-DPP-A-PY) using pyrene as a donor and diketopyrrolopyrrole as an acceptor were successfully synthesized and characterized. (HD/OD)-DPP-A-PY exhibited planar back-bone, conjugation extension, enhanced light absorption, and low HOMO energy level. Combined with the advanced properties, solution-processed OSCs based on a blend of HD-DPP-A-PY as a donor and [6,6]-phenyl-C₇₁-butyric-acid-methyl-ester (PC₇₀BM) as an acceptor exhibited PCEs as high as 3.15%.     

Efficient bilayer heterojunction polymer solar cells with bumpy donor–acceptor interface formed by facile polymer blend

We demonstrated a facile method for the fabrication of bilayer polymer solar cells with a controlled heterojunction structure via simple polymer blends. The spontaneous phase separation of poly(3-hexylthiophene)/polyethylene glycol blends provides a bumpy electron-donor layer with characteristic circular depressions. The diameter and depth of the circular depressions can be controlled by varying the PEG content of the blend. The deposition of -phenyl-C61-butyric acid methyl ester as an electron-acceptor layer then creates an interpenetrating donor–acceptor interface for bilayer heterojunction polymer solar cells. The bumpy morphology of the interface results in a significant enhancement in the power conversion efficiency over that of the bilayer polymer solar cells with a typical planar interface, which is mainly due to an increase of photocurrent. An estimation of the field-dependent possibility of charge separation indicates that charge extraction is more efficient than charge recombination in the bilayer devices and the increase in the interfacial area of solar cells with a bumpy-interface leads to generate more electron-hole pairs at the interface.     

Capacitance–voltage characteristics of P3HT:PCBM bulk heterojunction solar cells with ohmic contacts and the impact of single walled carbon nanotubes on them

Capacitance–voltage (C–V) characteristics of P3HT:PCBM devices of two different thicknesses are correlated with current density–voltage (J–V) characteristics. The rising portion of the C–V characteristics coincides with the exponential current density below the built-in voltage. The negative capacitance (NC) of these devices is a low frequency phenomenon and it occurs in trap-free space charge limited current (SCLC) regime. The onset frequencies of NC for devices with and without SWNTs also do not follow direct relation with effective mobility. The NC in thin devices has non-monotonic change with voltage for thin devices showing that interface state kinetics can be the reason for its occurrence. The NC of thick devices, on the other hand, increases monotonically with voltage showing that bulk properties dominate in these. Addition of SWNTs to these devices for efficiency improvement does not modify their built-in voltage. Also, the SWNTs do not affect the forward NC behaviour. However, the devices containing SWNTs show NC in reverse bias also which has different frequency dependence with voltage. The reverse bias NC is attributed to the large non-linear reverse current by charge injection into the additional energy levels introduced by SWNTs.     

High open-circuit voltage of the solution-processed organic solar cells based on benzothiadiazole–triphenylamine small molecules incorporating π-linkage

Two novel small molecular photovoltaic (PV) materials, BDPTBT and BDATBT were designed and synthesized, consisting of 5,6-bis-(octyloxy)benzo[c][1,2,5]thiadiazole (DOBT) as electron-withdrawing core (A), and triphenylamine (TPA) as electron-donating side group (D). Moreover, the benzene and ethynylbenzene as π-linkage were introduced to form donor–π-acceptor–π-donor (D–π-A–π-D) typed molecular structures, respectively. To fully investigate the linkage effect of a series of small molecules, two reference compounds BDCTBT and BDETBT were also studied systematically, consisting of 2-phenylacrylonitrile and styrene as π-linkage, respectively. As a result, the π-linkage units, benzene, styrene, ethynylbenzene and 2-phenylacrylonitrile played an important role in modifying molecular structure and improving PV performance. Bulk heterojunction (BHJ) solar cells based on BDPTBT/PC₆₁BM and BDATBT/PC₆₁BM yielded the power conversion efficiencies (PCEs) of 2.99% and 2.03%, respectively. Notably, BDATBT based device showed a high open-circuit voltage (Voc) of 1.03 V. Compared to the results we have reported previously, the reference devices based on BDCTBT/PC₆₁BM and BDETBT/PC₆₁BM with the optimized weight ratio showed dramatically enhanced PCEs of 4.84% and 3.40%, respectively, and BDCTBT based device showed a high Voc of 1.08 V. To our knowledge, the Voc of 1.08 V is the highest voltage reported to date for devices prepared from solution-processed small-molecule-donor materials, and the PCE of 4.84% is the highest efficiency reported so far for D–A–D-typed benzothiadiazole (BT)–TPA based solution-processed small molecules PV devices.     

Improved power conversion efficiency by insertion of RGO–TiO2 composite layer as optical spacer in polymer bulk heterojunction solar cells

We report that the power conversion efficiency (PCE) can be enhanced in polymer bulk heterojunction solar cells by inserting an interfacial electron transporting layer consisting of pristine TiO₂ or reduced graphene oxide–TiO₂ (RGO–TiO₂) between the active layer and cathode Al electrode. The enhancement in the PCE has been analyzed through the optical absorption, current–voltage characteristics under illumination and estimation of photo-induced charge carrier generation rate. It was found that either TiO₂ or RGO–TiO₂ interfacial layers improve the light harvesting, as well as the charge extraction efficiency, acting as a blocking layer for holes, and also reducing charge recombination. The combined enhancement in light harvesting property and charge collection efficiency improves the PCE of the organic solar cell up to 4.18% and 5.33% for TiO₂ and RGO–TiO₂ interfacial layer, respectively, as compared to a value of 3.26% for the polymer solar cell without interfacial layer.     

Novel D–A–π–A carbazole dyes containing benzothiadiazole chromophores for dye-sensitized solar cells

New D–A–π–A carbazole dyes containing benzothiadiazole chromophores were designed and synthesized for application in dye-sensitized solar cells (DSSCs). The light-harvesting capabilities and photovoltaic performance of these dyes were investigated systematically through comparison of different π-bridges and acceptors. Compared with thiophene bridge, benzene bridge provides improved IPCE and V_OC, which leads to better photoelectricity conversion efficiency. Dyes with cyanoacetic acid acceptor display superior photovoltaic properties though with shorter absorption maximum and lower molar absorption coefficient compared with dyes with rhodanine acetic acid acceptor. Therefore, dye with benzene bridge and cyanoacetic acid acceptor shows the most efficient photoelectricity conversion efficiency and has the maximum η value of 5.40% (V_OC = 710 mV, J_SC = 10.99 mA/cm², and ff = 0.71) under simulated AM 1.5 irradiation (100 mW/cm²).     

Novel perylene diimide based polymeric electron-acceptors containing ethynyl as the π-bridge for all-polymer solar cells

We designed and synthesised two n-type donor-acceptor copolymers based on perylenediimide (PDI). These copolymers contained an ethynyl moiety as the π-bridge between PDI and the electron-donating unit of either benzodithiophene (BDT) or fluorine (F), with the corresponding polymers denoted as PPDI-BDT and PPDI-F, respectively. The molecular geometry and the optical and electrochemical properties of the polymers were affected by the electron-donating unit. Both copolymers exhibited relatively broad light-absorption profiles and deep lowest unoccupied molecular orbital energy levels of about −3.8 eV. All-polymer solar cells (all-PSCs) were fabricated using these copolymers as the electron-accepting material and a medium-bandgap conjugated polymer, PBDB-T, as the electron-donating material. The resulting all-PSC based on PPDI-F exhibited a power conversion efficiency of 5.09%, which is significantly higher than that of the device fabricated using PPDI-BDT (2.73%). This improvement was attributed to the higher open-circuit voltage, greater charge carrier mobility and more effective charge transfer of the PBDB-T:PPDI-F blend film. These results suggest that the developed n-type PDI-based copolymers are promising candidates as electron-accepting materials for the construction of high-performance all-polymer solar cells.     

Enhancing efficiency for additive–free blade–coated small–molecule solar cells by thermal annealing

Blade coating was successfully applied to realise high-efficiency small-molecule organic solar cells (OSCs) with a solution-processed active layer comprising a small organic molecule DR3TBDTT with a benzo[1,2–b:4,5–b′]dithiophene (BDT) unit as the central building block as the donor and [6,6]–phenyl–C71–butyric acid methyl ester (PC₇₁BM) as the acceptor. Using chloroform as the solvent, a DR3TBDTT/PC₇₁BM blend active layer without an additive was effectively formed through blade coating. The power conversion efficiency (PCE) of small organic molecule solar cells was enhanced by 3.7 times through thermal annealing at 100 °C. This method produces OSCs with a high PCE of up to 6.69%, with an open circuit voltage (V_oc) of 0.97 V, a short-circuit current density (J_sc) of 12.60 mA/cm², and a fill factor (FF) of 0.55.     

Triarylamine-based dual-function coadsorbents with extended π-conjugation aryl linkers for organic dye-sensitized solar cells

Triarylamine-based dual-function coadsorbents containing a carboxylic acid acceptor linked by extended π-conjugation aryl linkers (e.g., phenylene: HC-A3, naphthalene: HC-A4 and anthracene: HC-A5) were newly designed and synthesized. They were used as coadsorbents in organic dye-sensitized solar cells (DSSCs) based on a porphyrin dye (hexyloxy-biphenyl-ZnP-CN-COOH (HOP)). For comparison, the π-conjugated phenyl linker (HC-A3) previously developed by our group was also used as a coadsorbent. The structural effects on the photophysical and electrochemical properties and DSSC performance were systematically investigated. As a result, the DSSCs based on HC-A4 and HC-5 displayed power conversion efficiencies (PCEs) of 8.2% and 5.1%, respectively, while the HC-A3-based DSSC achieved a PCE of 7.7%. In the case of HC-A4, both the short-circuit photocurrent densities (J_sc) and open-circuit voltages (V_oc) of DSSCs were simultaneously improved to a large extent due to the more effective prevention of π−π stacking of organic dye molecules and the better light-harvesting effect at short wavelengths. The HC-A5-based DSSC exhibited a much lower short-circuit current (J_sc) and open-circuit voltages (V_oc) compared to the HC-A4-based DSSC, due to the fact that the dihedral angle of the π-conjugated linkers was too high for electron injection into the TiO₂ conduction band (CB) level. This had a reduced effect on preventing the π−π stacking of dye molecules, resulting in lower J_sc and V_oc values.     

Novel dithieno[3,2-b:2′,3′-d]pyrrole-based organic dyes with high molar extinction coefficient for dye-sensitized solar cells

Three new metal-free organic dyes FD1–3 with a planar dithieno[3,2-b:2′,3′-d]pyrrole unit as linker were synthesized and used for dye-sensitized solar cells with high molar extinction coefficients. In this work, dithieno[3,2-b:2′,3′-d]pyrrole was employed as π-conjugated bridge to construct A–π–d–π–A organic dyes, where 9,9-dihexyl-9H-fluorene was used as a donor, and cyanoacrylic acid as an electron acceptor. For a typical device, a solar energy conversion efficiency (η) of 6.36% based on FD2 was achieved under simulated AM 1.5 solar irradiation (100 mW cm^?2) with a short-circuit photocurrent density (J_sc) of 13.76 mA cm^?2, an open-circuit voltage (V_oc) of 669 mV, a fill factor (ff) of 0.691. The results suggest that the organic dye with a functionalized dithienopyrrole unit is a promising candidate for DSSCs due to its high molar extinction coefficients.     

Benzo[1,2-b:4,5-b′]dithiophene and benzotriazole based small molecule for solution-processed organic solar cells

A novel deep HOMO A₁-π-A₂-D-A₂-π-A₁ type molecule (D(CATBTzT)BDT), which terminal electron-withdrawing octyl cyanoacetate group is connected to a benzo[1,2-b:4,5-b′]dithiophene (BDT) core through another electron-accepting benzotriazole block, has been synthesized, characterized, and employed as electron donor material for small molecule organic solar cells (SM-OSCs). By simple solution spin-coating fabrication process, D(CATBTzT)BDT/PC₆₁BM based OSCs exhibit a power conversion efficiency (PCE) of 3.61% with a high open-circuit voltage of 0.93 V. The D(CATBTzT)BDT based solar cells device also can show high FF of 72% with PCEs of 2.31% which is one of the best FF results for solution-processed SM-OSCs.