A Thieno[3,4-c]pyrrole-4,6-dione-Based Copolymer for Efficient Solar Cells

N-Alkylthienopyrroledione versus benzothiadiazole pulling units in push–pull copolymers used for photovoltaic applications: density functional theory study

Low-band-gap push–pull copolymers are promising donor materials for bulk heterojunction organic solar cells.

Amide-bridged terphenyl and dithienylbenzene units for semiconducting polymers

Semiconducting polymers with new “electron-neutral” amide-bridged building units were designed, synthesized, and studied as materials for organic devices.

Novel wide band gap polymers based on dithienobenzoxadiazole for polymer solar cells with high open circuit voltages over 1 V

Just by replacing sulfur with oxygen atom, the V_oc of PBDT-fDTBO based PSCs is 0.2 V higher than PBDT-fDTBT based devices.

All polymer solar cells with diketopyrrolopyrrole-polymers as electron donor and a naphthalenediimide-polymer as electron acceptor

Diketopyrrolopyrrole-polymers and N2200 were found to be highly miscible, which induced low efficiencies in all-polymer solar cells.

A pentacyclic building block containing an azepine-2,7-dione moiety for polymer solar cells

A pentacyclic building block containing an azepine-2,7-dione moiety (TTA) was developed. PTTABDT:PC₇₁BM solar cells gave a higher PCE of 5.46%, resulting from higherV_ocand ideal morphology.

N. A, B. H. Rational design of benzodithiophene based conjugated polymers for better solar cell performance

We present a concise review of conjugated polymers based on benzodithiophenes (BDTs) for high-performance polymer solar cells (PSCs).

A simple strategy to the side chain functionalization on the quinoxaline unit for efficient polymer solar cells

A new tetrafluoridequinoxaline electron accepting block-ffQx from a quinoxaline core was designed. A new copolymer (PBDTT-ffQx) was synthesized from tetrafluoridequinoxaline and benzodithiophene and a high PCE of 8.6% was obtained.

TPD-based polythiophene derivatives with higher Vocfor polymer solar cells

A new D–A copolymer photovoltaic donor material,PTPD-DFDT, with a simpler structure and easier synthesis, was synthesized, and the PSCs based onPTPD-DFDT/PC₇₁BM demonstrate a higherV_ocof 0.96 V and a PCE of 5.52%.

Low bandgap semiconducting polymers for polymeric photovoltaics

This review highlights the design rules for low bandgap semiconducting polymers, with the overview of their applications in polymer solar cells and polymer photodetectors.

2,1,3-Benzooxadiazole, thiophene and benzodithiophene based random copolymers for organic photovoltaics: thiophene versus thieno[3,2-b]thiophene as π-conjugated linkers

Influence of spacers on the optoelectronic properties of polymers.

Theoretical investigations on enhancing the performance of terminally diketopyrrolopyrrole-based small-molecular donors in organic solar cell applications

Diketopyrrolopyrrole (DPP)-based small molecules with acceptor-core-acceptor (A-core-A) type as donor materials have been used successfully in organic solar cells (OSC). In this work, based on the DPP-core-DPP type molecule SM1 consisting of a DPP unit as acceptor and benzene as the core, we replaced the benzene core with more electron-withdrawing groups in SM1 and further designed four new small-molecular donors (SM2-SM5) in order to improve the electrical properties, optical absorption and performance in OSC applications. The calculated results indicate that the designed small-molecular donors SM2-SM5 exhibit better performances in comparison with SM1, such as lower highest occupied molecular orbital (HOMO), narrower energy gap, larger absorption range, better electronic transfer between donor and acceptor and higher hole mobility. Moreover, the decreased HOMO levels and transition energy of small-molecular donors in OSC applications play an important role in the parameters of open-current voltage, fill factor and short-circuit current. Consequently, adjusting the electron-deficient ability of cores in DPP-core-DPP type small-molecular donors is an efficient approach that can be used to obtain high-efficiency DPP-based small-molecular donors for OSC applications. Graphical Abstract The designed small-molecules with good electronic and photophysical properties will act as a promising donor candidate for organic solar cell applications. Moreover, The decreased HOMO levels and transition energy of small-molecular donors in OSC applications play an important role in the parameters of open-current voltage, fill factor and short-circuit current.

Effect of Polymer Side Chains on Charge Generation and Disorder in PBDTTPD Solar Cells

The effect of polymer side chains on device performance was investigated for PBDT(EtHex)-TPD(Oct):PC70BM and PBDT(EtHex)-TPD(EtHex):PC70BM BHJ solar cells. Going from a linear side chain on the polymer's acceptor moiety to a branched side chain was determined to have a negative impact on the overall device efficiency, because of significantly reduced short-circuit current (J(sc)) and fill factor (FF) values. Sub-bandgap external quantum efficiency (EQE) and transient photoluminescence (PL) measurements showed more-efficient carrier generation for the polymer with linear side chains, because of a higher degree of charge-transfer (CT) state delocalization, leading to more-efficient exciton dissociation. Furthermore, the increase in π-π stacking distance and disorder for the bulkier ethylhexyl side chain were shown to result in a lower hole mobility, a higher bimolecular recombination, and a higher energetic disorder. The use of linear side chains on the polymer's acceptor moiety was shown to promote photogeneration, because of more-effective CT states and favorable carrier transport resulting in improved solar cell performance.

The Impact of Grain Alignment of the Electron Transporting Layer on the Performance of Inverted Bulk Heterojunction Solar Cells

This report presents a new strategy for improving solar cell power conversion efficiencies (PCEs) through grain alignment and morphology control of the ZnO electron transport layer (ETL) prepared by radio frequency (RF) magnetron sputtering. The systematic control over the ETL's grain alignment and thickness is shown, by varying the deposition pressure and operating substrate temperature during the deposition. Notably, a high PCE of 6.9%, short circuit current density (J(sc)) of 12.8 mA cm(-2), open circuit voltage (V(oc)) of 910 mV, and fill factor of 59% are demonstrated using the poly(benzo[1,2-b:4,5-b']dithiophene-thieno[3,4-c]pyrrole-4,6-dione):[6,6]-phenyl-C(71) -butyric acid methyl ester polymer blend with ETLs prepared at room temperature exhibiting oriented and aligned rod-like ZnO grains. Increasing the deposition temperature during the ZnO sputtering induces morphological cleavage of the rod-like ZnO grains and therefore reduced conductivity from 7.2 × 10(-13) to ≈1.7 × 10(-14) S m(-1) and PCE from 6.9% to 4.28%. An investigation of the charge carrier dynamics by femtosecond (fs) transient absorption spectroscopy with broadband capability reveals clear evidence of faster carrier recombination for a ZnO layer deposited at higher temperature, which is consistent with the conductivity and device performance.

D-π-A-π-A Strategy to Design Benzothiadiazole-carbazole-based Conjugated Polymer with High Solar Cell Voltage and Enhanced Photocurrent

The theoretical calculations are used to find that D-π-A-π-A style conjugated polymer PC-TBTBT is more efficient for solar cells application than the D-π-A analog PC-TBT because the D-π-A-π-A structure has a narrower band gap and higher molar absorption coefficient and redshift spectrum. Motivated by the theoretical prediction, 5,6-bis(octyloxy)-2,1,3-benzothiadiazole and 2,7-carbazole are adopted to synthesize the D-π-A-π-A style PC-TBTBT (M(w) = 31.1 kDa) and D-π-A analog PC-TBT (M(w) = 87.5 kDa) by Suzuki coupling reaction. Experimental results confirm that D-π-A-π-A PC-TBTBT-based solar cell shows a power conversion efficiency (PCE) of 4.74% with high V(OC) of 0.99 V and enhanced J(SC) of 9.70 mA cm(-2). The PCE and J(SC) achieve improvements of 17% and 26%, respectively, compared to the D-π-A PC-TBT-based solar cell.

Connecting Direct C-H Arylation Reactions with Dye-Sensitized Solar Cells: A Shortcut to D-A-π-A Organic Dyes

A step-economical synthetic strategy is developed to target thieno[3,4-c]pyrrole-4,6-dione (TPD)-based D-A-π-A organic dyes for dye-sensitized solar cells (DSSCs). Through sequential Pd-catalyzed direct C-H (hetero)arylation reaction, synthesis of the push-pull-type small molecules is reduced from the traditional six steps to two steps. In this report, we focus on the optimization of the key C-H monoarylation of TPD by screening ligands, acid additives, bases, and solvents. The reaction proves versatile toward new D-A-π-A organic dyes with a variety of different donor groups, and several derivatives are efficiently prepared under optimum reaction conditions. The sensitive aldehyde functionality that is a required intermediate for conversion into anchoring groups for TiO2 is well tolerated. Based on our synthetic study, DSSCs are fabricated and characterized using two designed sensitizers. The photovoltaic characterization of the devices affords an open-circuit voltage of 0.60-0.69 V, a short-circuit current density of 10.85-11.07 mA cm(-2), and a fill factor of 69.9-70.8 %, which corresponds to an overall power conversion efficiency of 4.61-5.33 %.

Continuous Flow Polymer Synthesis toward Reproducible Large-Scale Production for Efficient Bulk Heterojunction Organic Solar Cells

Organic photovoltaics (OPV) have attracted great interest as a solar cell technology with appealing mechanical, aesthetical, and economies-of-scale features. To drive OPV toward economic viability, low-cost, large-scale module production has to be realized in combination with increased top-quality material availability and minimal batch-to-batch variation. To this extent, continuous flow chemistry can serve as a powerful tool. In this contribution, a flow protocol is optimized for the high performance benzodithiophene-thienopyrroledione copolymer PBDTTPD and the material quality is probed through systematic solar-cell evaluation. A stepwise approach is adopted to turn the batch process into a reproducible and scalable continuous flow procedure. Solar cell devices fabricated using the obtained polymer batches deliver an average power conversion efficiency of 7.2 %. Upon incorporation of an ionic polythiophene-based cathodic interlayer, the photovoltaic performance could be enhanced to a maximum efficiency of 9.1 %.

Transient Photocurrent Response of Plasmon-Enhanced Polymer Solar Cells with Gold Nanoparticles

In this work, the transient photocurrent of the plasmon-enhanced polymer bulk heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-Phenyl C₆₁ butyric acid methyl ester (PCBM) is investigated. Two kinds of localized surface plasmon resonance (LSPR) enhanced devices were fabricated by doping the gold nanoparticles (Au NPs) into the anode buffer layer and inserting Au NPs between the anode buffer layer and the active layer. We probed the dynamics of the turn-on and turn-off responses to 400 μs square-pulse optical excitation from the 380 nm and 520 nm light-emitting diodes (LED) driven by an electric pulse generator. The transient photocurrent curves of devices with Au NPs at different positions and under different excitation wavelength are compared and analyzed. The charge trapping/detrapping processes that occurred at the interface of Au NPs and the active layer were observed; these exhibit an overshoot in the initial fast rise of photocurrent response. Our results show that the incorporating position of Au NPs is an important key factor to influence the transient photocurrent behaviors.

Highly Crystalline Low Band Gap Polymer Based on Thieno[3,4-c]pyrrole-4,6-dione for High-Performance Polymer Solar Cells with a >400 nm Thick Active Layer

Two thieno[3,4-c]pyrrole-4,6-dione (TPD)-based copolymers combined with 2,2'-bithiophene (BT) or (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TV) have been designed and synthesized to investigate the effect of the introduction of a vinylene group in the polymer backbone on the optical, electrochemical, and photovoltaic properties of the polymers. Although both polymers have shown similar optical band gaps and frontier energy levels, regardless of the introduction of vinylene bridge, the introduction of a π-extended vinylene group in the polymer backbone substantially enhances the charge transport characteristics of the resulting polymer due to its strong tendency to self-assemble and thus to enhance the crystallinity. An analysis on charge recombination in the active layer of a solar cell device indicates that the outstanding charge transport (μ = 1.90 cm(2)·V(-1)·s(-1)) of PTVTPD with a vinylene group effectively suppresses the bimolecular recombination, leading to a high power conversion efficiency (PCE) up to 7.16%, which is 20% higher than that (5.98%) of the counterpart polymer without a vinylene group (PBTTPD). More importantly, PTVTPD-based devices do not show a large variation of photovoltaic performance with the active layer thickness; that is, the PCE remains at 6% as the active layer thickness increases up to 450 nm, demonstrating that the PTVTPD-based solar cell is very compatible with industrial processing.

Visualization of Hot Exciton Energy Relaxation from Coherent to Diffusive Regimes in Conjugated Polymers: A Theoretical Analysis

The unified coherent-to-diffusive energy relaxation of hot exciton in organic aggregates or polymers, which still remains largely unclear and is also a great challenge theoretically, is investigated from a time-dependent wavepacket diffusive approach. The results demonstrate that in the multiple time scale energy relaxation dynamics, the fast relaxation time essentially corresponds to the dephasing time of excitonic coherence motion, whereas the slow time is related to a hopping migration, and a suggested kinetic model successfully connects these two processes. The dependencies of those times on the initial energy and delocalization of exciton wavepacket as well as exciton-phonon interactions are further analyzed. The proposed method together with quantum chemistry calculations has explained an experimental observation of hot exciton energy relaxation in the low-bandgap copolymer PBDTTPD.

Donor-acceptor small molecules for organic photovoltaics: single-atom substitution (Se or S)

Two isostructural low-band-gap small molecules that contain a one-atom substitution, S for Se, were designed and synthesized. The molecule 7,7'-[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene]bis[6-fluoro-4-(5'-hexyl-2,2'-bithiophen-5-yl)benzo[c][1,2,5]thiadiazole] (1) and its selenium analogue 7,7'-[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene]bis[6-fluoro-4-(5'-hexyl-2,2'-bithiophen-5-yl)benzo[c][1,2,5]selenodiazole] (2) are both based on the electron-rich central unit benzo[1,2-b:4,5-b']dithiophene. The aim of this work was to investigate the effect of one-atom substitution on the optoelectronic properties and photovoltaic performance of devices. Theoretical calculations revealed that this one-atom variation has a small but measurable effect on the energy of frontier molecular orbital (HOMO and LUMO), which, in turn, can affect the absorption profile of the molecules, both neat and when mixed in a bulk heterojunction (BHJ) with PC71BM. The Se-containing variant 2 led to higher efficiencies [highest power conversion efficiency (PCE) of 2.6%] in a standard organic photovoltaic architecture, when combined with PC71BM after a brief thermal annealing, than the S-containing molecule 1 (highest PCE of 1.0%). Studies of the resulting morphologies of BHJs based on 1 and 2 showed that one-atom substitution could engender important differences in the solubilities, which then influenced the crystal orientations of the small molecules within this thin layer. Brief thermal annealing resulted in rotation of the crystalline grains of both molecules to more energetically favorable configurations.

Design and photovoltaic characterization of dialkylthio benzo[1,2-b:4,5-b′]dithiophene polymers with different accepting units

Three dialkylthio benzo[1,2-b:4,5-b′]dithiophene polymers were developed with tuned bandgaps exhibiting a highest PCE value of 5.63% obtained for the polymer solar cells.

Donor–acceptor conjugated polymers based on multifused ladder-type arenes for organic solar cells

In this review, we summarize the recent development of the multifused ladder-type conjugated building blocks for making donor–acceptor conjugated copolymers.

Design, synthesis and electronic properties of push–pull–push type dye

Alkyne conjugated push–pull–push type dye where alkyne π-spacer and push units monitor the electronic properties of the dye.

Synthesis of a benzotriazole bearing alternating copolymer for organic photovoltaic applications

Improvement of the efficiencyviabenzodithiphene based polymer in organic photovoltaics.

An Alternating Donor–Acceptor Conjugated Polymer Based on Benzodithiophene and [3,4-c]pyrrole-4,6-dione: Synthesis, Characterization, and Application in Photovoltaic Devices

The synthesis is described of a new alternating donor–acceptor semiconducting polymer based on an N-octylthieno[3,4-c]pyrrole-4,6-dione building block together with a newly designed 2,3-bis(2-ethylhexyl)thiophenylethynyl substituted benzodithiophene (BDT). The introduction of electron-rich thiophene units to BDT raises the highest occupied molecular orbital (HOMO) level of the conjugated polymer and the concomitant reduction of the bandgap enhances the harvesting of solar radiation. This modification also introduces less sterically demanding triple bonds, thereby potentially enabling more favourable molecular interactions and an extra dimension of conjugation perpendicular to the main polymer chain. The optoelectronic properties of this new conjugated polymer were evaluated using UV-visible absorption and fluorescence spectroscopy, photoelectron spectroscopy in air, photo-induced charge extraction by linearly increasing voltage (Photo-CELIV), and density functional theory calculations. The polymer absorbs broadly in the wavelength range 300–700 nm in solution and the solid state. The estimated HOMO and LUMO levels of −5.4 and −3.6 eV, respectively, correspond to a bandgap of 1.8 eV. Photovoltaic devices fabricated using the polymer as the active layer displayed power conversion efficiencies (PCEs) of up to 1 %. Photo-CELIV results provide evidence that rapid recombination and poor charge mobility are likely contributing factors to the relatively low PCE values observed.