Improving the nanoscale morphology and processibility for PCDTBT-based polymer solar cells via solvent mixtures

The effect of solvent mixtures on the morphologies of poly[N-9′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) blend films is investigated. 1,2,4-Trichlorobenzene (TCB) which is a good solvent for PCDTBT is selected to mix with chloroform (CF), chlorobenzene (CB) and o-dichlorobenzene (oDCB) for tuning the morphology of the PCDTBT:PC₇₁BM blend. It is found that formation of nanoscale phase separation with a fibrillar PCDTBT nanostructure of PCDTBT:PC₇₁BM blend which is favorable for exciton separation and charge carrier transport is strongly dependent on the solubility parameters of the solvent mixtures. A clearly defined nanoscale phase separation of the PCDTBT:PC₇₁BM blend can be obtained with TCB:CF mixture. The resulted morphology is similar to that produced with sole DCB solvent that is currently the best solvent for PCDTBT:PC₇₁BM blend solar cells. Moreover, the TCB:CF mixture demonstrates better solubility and processibility for PCDTBT:PC₇₁BM blend and allows us to prepare thick active layer that is required in large-area roll-to-roll process. The polymer solar cells with 250 nm- thick active layer are fabricated by using TCB:CF solvent mixture and the power conversion efficiency of the devices reaches 6.45%. A highest short-circuit current of 13.6 mA/cm² is achieved due to enhanced optical absorption of thick active layer.     

Solvent–vapor induced morphology reconstruction for efficient PCDTBT based polymer solar cells

This paper reports polymer solar cells with a 7% power conversion efficiency (PCE) based on bulk heterojunction (BHJ) composites of the alternating co-polymer, poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT), and the fullerene derivative [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM). As confirmed by transmission electron microscopy, solvent–vapor annealing (SVA) of the thin (70 nm) BHJ photoactive layer by exposure to chloroform vapor, for a short period of time (30 s) after deposition, leads to reconstructed nanoscale morphology of donor/acceptor domains, well-dispersed fullerene phase and effective photo-absorption of BHJ. Consequently, SVA-reconstructed devices with a PCDTBT:PC₇₁BM blend ratio of 1:5 (wt%) exhibit ～50% improvement in PCE, with short-circuit current J_sc = 15.65 mA/cm², open-circuit voltage V_oc = 0.87 V, and PCE = 7.03%, in comparison to those of the 1:4 (wt%) blends with SVA treatment.     

Efficient semitransparent organic solar cells with good color perception and good color rendering by blade coating

This paper proposes high efficiency semitransparent organic solar cells (OSCs) with good color perception and good color rendering using blade coating technique. We investigate four different polymer blends and first fabricate small area devices with active area of 0.04 cm², followed by large area devices with active area of 10.8 cm². Two of the polymer blends, 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene:6,6-phenyl C71-butyric acid methyl ester (PBDTTT-CT:PC₇₁BM) and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′] dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]:PC₇₁BM (PBDTTT-EFT:PC₇₁BM) show promising results. For small area devices, semitransparent PBDTTT-CT:PC₇₁BM and semitransparent PBDTTT-EFT:PC₇₁BM achieve a power conversion efficiency (PCE) of 5.2% (opaque PCE = 7.5%) and 5.6% (opaque PCE = 9.4%) respectively. For large area devices, they are found to produce a PCE of 3.8% (opaque PCE = 4.2%) and 5.3% (opaque PCE = 5.9%) respectively. Based on the CIE 1931 chromaticity diagram, semitransparent PBDTTT-CT:PC₇₁BM and semitransparent PBDTTT-EFT:PC₇₁BM are located very close to the standard illuminant D65, indicating good color perception. As for color rendering, they demonstrate high color rendering index (CRI) of 95.4 and 87.1 respectively. These combined high performances indicate high-quality transmitted light, which is suitable for window application.     

A star-shaped oligothiophene with triphenylamine as core and octyl cyanoacetate as end groups for solution-processed organic solar cells

A new star-shaped D–π–A molecule with triphenylamine (TPA) as core and donor unit, octyl cyanoacetate (CA) as end group and acceptor unit, and 2,2′-bithiophene vinylene (bTV) as π bridge, S(TPA-bTV-CA) was designed and synthesized for the application as donor materials in solution-processed bulk-heterojunction organic solar cells (OSCs). The compound is soluble in common organic solvents. The thermal, optical and electrochemical properties of the star molecule were studied. The OSC devices were fabricated by spin-coating the blend solution of the molecule as donor and PC₇₀BM as acceptor (1:3, w/w). The OSC device based on S(TPA-bTV-CA)/PC₇₀BM demonstrated a high open circuit voltage of 0.91 V, a short circuit current density of 4.64 mA/cm², a fill factor (FF) of 50%, corresponding to a power conversion efficiency of 2.1%, under the illumination of AM 1.5, 100 mW/cm².     

Active layer thickness effect on the recombination process of PCDTBT:PC71BM organic solar cells

We investigated the effect of active layer thickness on recombination kinetics of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM) based solar cells. Analysis of the fitted Lambert W-function of illuminated current density–voltage (J–V) characteristics revealed increased recombination processes with increased active layer thicknesses. The ideality factor extracted from PCDTBT:PCBM solar cells continuously increased from 1.89 to 3.88 when photoactive layer thickness was increased from 70 to 150 nm. We found that such increase in ideality factor is closely related to the defect density which is increased with increased photoactive layer thickness beyond 110 nm. Therefore, the different density of defect states in PCDTBT:PCBM solar cells causes the different recombination paths where solar cells with a thicker active layer (?110 nm) are considered to undergo coupled trap-assisted recombination processes while single-defect trap-assisted recombination is dominant for thinner (70–90 nm) PCDTBT:PCBM solar cells. As a result, we found that the optimal efficiencies of PCDTBT:PC₇₁BM solar cells were limited to the active layers between 70 and 90 nm. Particularly, when PCDTBT:PC₇₁BM solar cells were optimized with an active layer thickness of 70 nm, energy conversion efficiency reached 6.5% while an increase in thickness led to the reduction of efficiency to 4.7% at 133 nm but then an increase to 5.02% at 150 nm.     

Synthesis,optical and electrochemical properties new donor–acceptor (D–A) copolymers based on benzo[1,2-b:3,4-b′:6,5-b″] trithiophene donor and different acceptor units: Application as donor for photovoltaic devices

Two new conjugated D–A polymers P3 (PBTT-d-BTT) and P4 (PBTT-d-TPD) based on same benzo[1,2-b:3,4-b′:6,5-b″] trithiophene (BTT) donor and different acceptors monomers 5,8-dibromo-2-dodecanoylbenzo[1,2-b:3,4-b′:6,5-b″] trithiophene (d-BTT), and 1,3-dibromo-5-(2-ethylhexyl)thieno[3,4]pyrrol-4,6-dione (d-TPD) respectively, were synthesized by Stille cross-coupling reaction and characterized by gel permeation chromatography (GPC), ¹H NMR, UV–Vis absorption, thermal analysis and electrochemical cyclic voltammetry (CV) tests. Photovoltaic properties of the polymers were studied by using the polymers as donor and PC₇₁BM as acceptor with a weight ratio of polymer:PC₇₁BM 1:1, 1:2 and 1:2.5. The optimized photovoltaic device was fabricated with an active layer of a blend P3:PC₇₁BM and P4:PC₇₁BM with a blend ratio of 1:2 showed PCE 3.16% and 2.42%, respectively under illumination of AM 1.5 at 100 mW/cm² with solar simulator. The PCE of the device based on P3:PC₇₁BM processed with DIO/o-DCB has been further improved up to 4.64% with J_sc of 10.52 mA/cm² and FF of 0.58 attributed to the increase in crystalline nature of active layer and more balanced charge transport in the device, induced by DIO additive.     

Effect of blend layer thickness on the dielectric response of PCDTBT:PC71BM-based bulk heterojunction solar cells

Bulk heterojunction solar cells based on a blend of poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM), were studied. The organic photoactive layers were spin coated onto a poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT-PSS) interfacial layer at speeds of 600 and 2000 rpm. The molecular structure of PCDTBT, PC₇₁BM, and the PCDTBT:PC₇₁BM blend was investigated using Fourier-transform IR (FTIR) spectroscopy, which confirmed the absence of interactions between the individual components of the composite. The dielectric properties of PCDTBT:PC₇₁BM-based solar cells were studied under illumination by means of impedance analysis. The dielectric constant, impedance, and ac conductance were analyzed as a function of frequency at different bias voltages close to the open circuit voltage (Voc). We found that the dielectric constant, dielectric loss, and conductance increased with increasing PCDTBT:PC₇₁BM thickness. Impedance spectroscopy analysis revealed decreases in charge recombination and the resistance of the whole device with increasing spin coating speed for the active layer. Moreover, an increase in recombination resistance for the solar cells was observed close to V_OC.     

Correlating Structure with Function in Thermally Annealed PCDTBT:PC70BM Photovoltaic Blends

A range of optical probes are used to study the nanoscale‐structure and electronic‐functionality of a photovoltaic‐applicable blend of the carbazole co‐polymer poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and the electronic accepting fullerene derivative (6,6)‐phenyl C₇₀‐butyric acid methyl ester (PC₇₀BM). In particular, it is shown that the glass transition temperature of a PCDTBT:PC₇₀BM blend thin‐film is not sensitive to the relative blend‐ratio or film thickness (at 1:4 blending ratio), but is sensitive to casting solvent and the type of substrate on which it is deposited. It is found that the glass transition temperature of the blend reduces on annealing; an observation consistent with disruption of π–π stacking between PCDTBT molecules. Reduced π–π stacking is correlated with reduced hole‐mobility in thermally annealed films. It is suggested that this explains the failure of such annealing protocols to substantially improve device‐efficiency. The annealing studies demonstrate that the blend only undergoes coarse phase‐separation when annealed at or above 155 °C, suggesting a promising degree of morphological stability of PCDTBT:PC₇₀BM blends.     

Highly efficient thieno[3,4-c]pyrrole-4,6-dione-based solar cells processed from non-chlorinated solvent

To obtain high performance bulk heterojunction organic solar cells, the selection of solvents to prepare the donor/acceptor blend is as important as the choice of the donor/acceptor materials themselves. State-of-the-art lab-scale polymer solar cells have evolved around chlorinated solvents such as chloroform, chlorobenzene and o-dichlorobenzene. However, for large scale applications, benign processing solvents may become inevitable. In this work, we used a mixture of Xylenes (a chlorine-free solvent), methyl naphthalene (MeN) and 1,8-diiodoctane (DIO) to modulate the nano-scale morphology of poly(4,4-bis(2-ethylhexyl)-dithieno[3,2-b:2′,3′-d]silole-alt-1,3-(5-octylthieno[3,4-c]pyrrole-4,6-dione) (PDTSTPD)/PCBM blend, one of the most efficient active layer in polymeric solar cell. Power conversion efficiencies up to 5.5% (with PC₆₁BM) and 6.2% (with PC₇₁BM) were obtained for photovoltaic devices with an active area of 1.0 cm².     

Air-processed high performance ternary blend solar cell based on PTB7-Th:PCDTBT:PC70BM

Ternary bulk heterojunctions (BHJs) are promising candidates that can improve the power conversion efficiencies (PCEs) of organic solar cells (OSCs). In this paper, a ternary OSC with two donors, including one wide bandgap polymer poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), one low bandgap polymer 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] (PTB7-Th), and one acceptor [6,6]-phenyl C70 butyric acid methyl ester (PC₇₀BM), is fabricated in atmospheric conditions. By incorporating a 20% content of PCDTBT, an optimized PCE of 7.86% for ternary OSC is characterized by a short-circuit current density (J_sc) of 15.21 mA cm⁻², a fill factor of 69.70% and an open-circuit voltage (V_oc) of 0.74 V. The V_oc values increased steadily from 0.73 to 0.86 V as the increase of PCDTBT fraction, which indicates that the V_oc of ternary OSC is not limited by the smallest one of the corresponding binary OSC. We show that the J_sc of the ternary OSC is better than those of the binary OSC in virtue of the complementary polymer absorption and cascade energy levels, as well as optimized morphology of the ternary system. Furthermore, the lifetime of the devices with PCDTBT is greatly enhanced. This work indicates that two donors (PTB7-Th/PCDTBT) ternary BHJs system provide a simple and effective method to improve the performance and also the stability of OSCs.     

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.     

Use of benzothiadiazole–triphenylamine amorphous polymer for reproducible performance of polymer–fullerene bulk-heterojunction solar cells

An amorphous polymer, poly(BTD-TPA), which consists of benzothiadiazole and triarylamine units, can be successfully utilized to fabricate bulk heterojunction (BHJ) organic photovoltaics (OPVs), and the OPV performance can be demonstrated to be independent of the casting solvent or thermal annealing temperature. The OPV based on poly(BTD-TPA):PC₇₀BM (1:4) that was fabricated using chloroform (boiling point of 61 °C) and annealed at 60 °C for 10 min exhibited a power conversion efficiency (PCE) of 2.81% under simulated solar irradiation through an air mass of 1.5 at 100 mW cm⁻². On the other hand, the OPV fabricated using o-dichlorobenzene (boiling point of 181 °C) and annealed at 110 °C for 10 min exhibited a PCE of 2.65%. Almost the same PCEs and incident photon to current conversion efficiencies (IPCEs) were obtained in both OPVs. The use of an amorphous film of poly(BTD-TPA) in the fabrication of OPVs offers great advantages over the use of a polycrystalline film of regioregular poly(3-hexylthiophene) (P3HT) in terms of high reproducibility of the OPV performance.     

A new diketopyrrolopyrrole-based co-polymer for ambipolar field-effect transistors and solar cells

A new thieno[3,2-b]thiophenediketopyrrolopyrrole-benzo[1,2-b:4,5-b′]dithiophene based narrow optical gap co-polymer (PTTDPP-BDT) has been synthesized and characterized for field-effect transistors and solar cells. In field-effect transistors the polymer exhibited ambipolar charge transport behaviour with maximum hole and electron mobilities of 10⁻³ cm² V⁻¹ s⁻¹ and 10⁻⁵ cm² V⁻¹ s⁻¹, respectively. The respectable charge transporting properties of the polymer were consistent with X-ray diffraction measurements that showed close molecular packing in the solid state. The difference in hole and electron mobilities was explained by density functional theory calculations, which showed that the highest occupied molecular orbital was delocalized along the polymer backbone with the lowest unoccupied molecular orbital localized on the bis(thieno[3,2-b]thiophene)diketopyrrolopyrrole units. Bulk heterojunction photovoltaic devices with the fullerene acceptor PC₇₀BM were fabricated and delivered a maximum conversion efficiency of 3.3% under AM1.5G illumination.     

Air-stability and bending properties of flexible organic field-effect transistors based on poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]

We have fabricated flexible field-effect transistors (FETs) using poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], PCDTBT, as an active channel, poly(methyl methacrylate) (PMMA) as gate dielectric and biaxially oriented polyethyleneterephthalate (BOPET) as supporting substrate. The output and transfer characteristics of the devices were measured as a function of channel length. It has been observed that various OFET parameters viz. on–off ratio (∼10⁵), mobility (μ ∼ 10⁻⁴ cm² V⁻¹ s⁻¹), threshold voltage (V_th ∼ −14 V), switch-on voltage (V_so ∼ −6 V), subthreshold slope (S ∼ 7 V/decade) and trap density (N_it ∼ 10¹⁴ cm⁻² V⁻¹) are almost independent of the channel length, which suggested a very high uniformity of the PCDTBT active layer. These devices were highly stable under atmospheric conditions (temperature: 20–35 °C and relative humidity: 70–85%), as no change in mobility was observed on a continuous exposure for 70 days. The studies on the effect of strain on mobility revealed that devices are stable up to a compressive or tensile strain of 1.2%. These results indicate that PCDTBT is a very promising active layer for the air stable and flexible FETs.     

Solution-processable star-shaped photovoltaic organic molecules based on triphenylamine and benzothiadiazole with longer pi-bridge

Two solution-processable star-shaped D-π-A organic molecules with triphenylamine (TPA) as donor unit, benzothiadiazole (BT) as acceptor unit and 4-hexyl-thienylenevinylene as pi conjugated bridge, S(TPA-TBTT) and S(TPA-TBTT-TPA), have been designed and synthesized for the application as donor materials in bulk-heterojunction organic solar cells (OSCs). The two molecules possess broader absorption from 350 to 700 nm benefitted from the longer pi-bridge in the molecules but weaker absorbance and poorer solubility in comparison with their corresponding organic molecules with shorter vinylene pi-bridge. The OSC based on S(TPA-TBTT): PC₇₀BM (1:3, w/w) exhibited J_sc of 6.41 mA/cm², V_oc of 0.75 V, FF of 39.0% and power conversion efficiency of 1.90%, under the illumination of AM 1.5, 100 mW/cm².     

Energy levels in dilute-donor organic solar cell photocurrent generation: A thienothiophene donor molecule study

To investigate photocurrent generation mechanisms in these organic solar cells (OSCs), we design and synthesize four thienothiophene (TT)-based small-molecule donors with the highest occupied molecular orbital (HOMO) levels varying from −6.4 eV to −5.1 eV, which span across the HOMO value of the [6,6]-phenyl-C70-butyric acid methyl ester (PC₇₁BM) acceptor. We measure TT-based donor:PC₇₁BM films’ electronic and optical properties, OSC current density-voltage characteristic, and external quantum efficiency, and perform density functional theory (DFT) calculations. Our results show that photocurrent generation depends strongly on the substitutions of the center TT groups, cyano (-CN) versus hexyloxy (-OHex). With 1 wt% donor, TTOHex:PC₇₁BM devices produce seven times, increasing to twelve times for 5 wt % donor, higher photocurrent than neat PC₇₁BM devices. In contrast, TTCN:PC₇₁BM devices do not generate additional photocurrent even with 10 wt% donor. The photocurrent generation in TT-based donor:PC₇₁BM devices depends critically on the HOMO value of the donor molecule with respect to that of PC₇₁BM, indicating the importance of type II energy level alignment to facilitate exciton dissociation at the donor-acceptor interface. The photovoltage of all TT:PC₇₁BM devices are comparable to neat PC₇₁BM devices, 0.85–0.90 V, with a low voltage loss due to non-radiative recombination. The fill factor of TTOHex:PC₇₁BM devices are low due to the low hole mobility, ~10⁻⁸ cm²/V. Following exciton dissociation, hole transport is analyzed according to three possible mechanisms: tunneling, percolation pathways, and hole back transfer. We find that the hole back transfer mechanism can explain all experimental results and therefore is the primary hole transport mechanism for photocurrent generation in TT-based donor:PC₇₁BM dilute-donor OSCs.     

Control of the active layer nanomorphology by using co-additives towards high-performance bulk heterojunction solar cells

In this work, two high boiling-point solvents (1-chloronaphthalene (CN) and 1,8-diiodooctane (DIO)) were utilized as co-additives in ortho-dichlorobenzene (ODCB) and chlorobenzene (CB) solutions to fine tune the donor and acceptor domains in the bulk heterojunction (BHJ) of poly(benzo[1,2-b:4,5-b′]dithiophene-alt-thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) and fullerene derivatives. A power conversion efficiency of 7.1% and a fill factor up to 70% were obtained for solar cells with active area of 1 cm² when using [6,6]-phenyl C61-butyric acid methyl ester (PC₆₀BM) as acceptor, suggesting that an optimized morphology was achieved.     

Dicyano-functionalized chlorophyll derivatives with ambipolar characteristic for organic photovoltaics

Two ambipolar chlorophyll derivatives, namely, 3²,3²-dicyano-pyropheophorbide-a (Chl-1) and methyl 13¹-deoxo-13¹-(dicyanomethylene) pyropheophorbide-a (Chl-2), were synthesized for use as either the electron acceptor or the electron donor in organic planar-heterojunction solar cells. Despite the higher electron mobilities of these chlorophyll derivatives compared with their hole mobilities, devices using them as the electron donor with fullerene C₇₀ give much better photovoltaic performance than when they are used as the electron acceptor with copper phthalocyanine. In these Chl-based solar cells, the energy gap between the LUMO levels of the donor and acceptor molecules substantially affects the charge separation and resultant photocurrent and photovoltaic performance. The highest solar energy-to-electricity conversion efficiency of up to 2.3% has been achieved using the Chl-2/C₇₀ solar cell, under AM1.5 solar illumination (100 mW/cm²) after thermal annealing of the device. It was also confirmed that the electron mobility of blend films containing Chls and fullerene derivative PC₇₀BM was determined not only by the electron mobility of PC₇₀BM but also by that of Chls.     

Dithienopyrrole-benzodithiophene based donor materials for small molecular BHJSCs: Impact of side chain and annealing treatment on their photovoltaic properties

Two small molecular organic materials denoted as ICT1 and ICT2 with A-D₁-D₂-D₁-A architecture have been synthesized and their thermal, photo-physical, electrochemical and photovoltaic properties are explored. Synthesized materials have n-butylrhodanine acceptor (A), dithienopyrrole (DTP) (D₁) and benzodithiophene (BDT) (D₂) (Alkoxy BDT and alkylthiophene BDT, respectively for ICT1 and ICT2) donor moieties. Both the materials have good solubility (up to 25 mg/mL) in most common organic solvents and have excellent thermal stability with the decomposition temperature (T_d) as 348 and 382 °C, respectively for ICT1 and ICT2. Both ICT1 and ICT2 have broad and intense visible region absorption (molar excitation coefficient is 1.71 × 10⁵ and 1.65 × 10⁵ mol⁻¹ cm⁻¹, respectively for ICT1 and ICT2) and have suitable HOMO and LUMO energy levels for PC₇₁BM acceptor. Bulk heterojunction solar cells with ITO/PEDOT:PSS/blend/Al structure are fabricated using these materials. The BHJSCs fabricated by spin cast of ICT1:PC₇₁BM and ICT2:PC₇₁BM (1:2 wt ratio) blend from chloroform showed power conversion efficiency (PCE) of 2.77% (J_sc = 6.84 mA/cm², V_oc = 0.92 V and FF = 0.44) and 3.27% (J_sc = 7.26 mA/cm², V_oc = 0.96 V and FF = 0.47), respectively. Annealing the active layer significantly improved the PCE of these BHJSCs to 5.12% (J_sc = 10.15 mA/cm², V_oc = 0.87 V and FF = 0.58) and 5.90% (J_sc = 10.68 mA/cm², V_oc = 0.92 V and FF = 0.60), respectively for ICT1 and ICT2 donors. The enhancement in the PCE is due to higher light harvesting ability of the active layer, better nanoscale morphology for efficient and balanced charge transport and effective exciton dissociation at the donor-acceptor interface.     

A crystalline D-π-A organic small molecule with naphtho[1,2-b:5,6-b′]dithiophene-core for solution processed organic solar cells

In this work, we have designed and synthesized a new naphtho[1,2-b:5,6-b′]dithiophene-containing enlarged π-conjugated donor–acceptor (D–A) small molecule, NDT(TTz)₂, for use in solution-processed organic photovoltaics. NDT(TTz)₂, which contains a thiophene-bridged naphtho[1,2-b:5,6-b′]dithiophene as the central fused core and triphenylamine-flanked thiophene thiazolothiazole as a spacer, was synthesized via sequential Suzuki and Stille coupling reactions. The thermal, physiochemical, and electrochemical properties of NDT(TTz)₂ have been evaluated by differential scanning calorimetry, thermogravimetry, UV–Vis spectroscopy, photoluminescence spectroscopy, X-ray diffraction, and cyclic voltammetry. As desired for photovoltaic applications, NDT(TTz)₂ possesses good solubility, thermal stability, and a well-ordered, π–π stacked, crystallinity. The optical band gap and HOMO level of NDT(TTz)₂ were determined to be 2.0 eV and −5.23 eV, respectively. In addition to organic thin film transistor studies, application of NDT(TTz)₂ to preliminary photovoltaic devices has also been investigated by fabricating solution-processed bulk heterojunction solar cells together with PC₇₁BM in a typical layered device structure, ITO/PEDOT:PSS/NDT(TTz)₂:PC₇₁BM/LiF/Al. Without extensive optimization of the device, NDT(TTz)₂ in these devices shows a maximum power conversion efficiency of 1.44% under AM 1.5 illumination at a 100 mW/cm² intensity.