Power Conversion Efficiency Of Polymer Solar Cells Research Articles

The selenium substitution of solvent additive enables efficient polymer solar cells with efficiency of 19.4 %

Solvent additives, which are widely used as versatile tools for refining active layer morphology, have been considered as important role in enhancing the power conversion efficiency (PCE) of polymer solar cells (PSCs). Herein, innovative solvent additive 2,5-dibromoselenophene (BrS), is devised and synthesized by substituting the sulfur atom in 2,5-dibromothiophene (BrT) with selenium atom. The facile polarization properties of selenium atoms enable selenophene to participate in intermolecular interactions with oxygen or sulfur atoms. Therefore, incorporating BrS into the blend solution leads to refined intermolecular interactions and molecular packing, and crystallinity, resulting in a refined bulk heterojunction (BHJ) morphology compared to using the solvent additives 1-chloronaphthalene (CN) and BrT. Consequently, the BrS outperform their BrT and CN in improving photon absorption, exciton dissociation, and charge collection properties of PSCs, yielding an enhanced PCE of 18.0 % in PM6:Y6 BHJ devices. Moreover, the BrS solvent additive exhibit general applicability in both non-fullerene small molecules PSCs and all-polymer PSCs. High efficiency of 18.8 %, 19.4 % and 18.1 % are obtained in PM6:L8-BO, D18:L8-BO, and PM6:PY-IT BHJ solar cells, respectively, which is rankable among the paramount performance reported to date. Our results highlight the potential of selenophene-based solvent additives as an observable pathway for nanomorphology of active layer and effective enhancement of PCE in PSCs.

Recent progress and prospects of dimer and multimer acceptors for efficient and stable polymer solar cells.

High power conversion efficiency (PCE) and long-term stability are essential prerequisites for the commercialization of polymer solar cells (PSCs). Small-molecule acceptors (SMAs) are core materials that have led to recent, rapid increases in the PCEs of the PSCs. However, a critical limitation of the resulting PSCs is their poor long-term stability. Blend morphology degradation from rapid diffusion of SMAs with low glass transition temperatures (Tgs) is considered the main cause of the poor long-term stability of the PSCs. The recent emergence of oligomerized SMAs (OSMAs), composed of two or more repeating SMA units (i.e., dimerized and trimerized SMAs), has shown great promise in overcoming these challenges. This innovation in material design has enabled OSMA-based PSCs to reach impressive PCEs near 19% and exceptional long-term stability. In this review, we summarize the evolution of OSMAs, including their research background and recent progress in molecular design. In particular, we discuss the mechanisms for high PCE and stability of OSMA-based PSCs and suggest useful design guidelines for high-performance OSMAs. Furthermore, we reflect on the existing hurdles and future directions for OSMA materials towards achieving commercially viable PSCs with high PCEs and operational stabilities.

Enhancing Efficiency and Stability of Polymer Solar Cells Based on CuI Nanoparticles as the Hole Transport Layer

Copper(І) iodide (CuI) is a very efficient inorganic p-type material and has a potential application in polymer solar cells (PSCs). However, the solution-processed CuI film tends to be rough surface morphology. Herein, CuI nanoparticles (NPs) were synthesized by water decomposing the CuI N,N-dimethylformamide (DMF) dispersion and used as hole transporting layers (HTLs) of PSCs with the configuration of ITO/CuI/P3HT:PC61BM/Ca/Al. As a result, a smooth and uniform CuI film was formed by spin coating from CuI NP acetonitrile solution. The root-mean-square roughness of the film reduced from 2.25 nm for spin coating from bulk CuI acetonitrile solution to 0.65 nm for CuI NPs. The power conversion efficiency of PSCs increased from 3.10% to 3.47% for CuI NP HTL. Moreover, the CuI NPs-based device demonstrated enhanced stability. The higher efficiency and better stability of CuI NPs-based PSCs should be attributed to uniform surface morphology and reducing exciton quenching of the CuI/the photoactive layer interface. This work indicates that CuI NPs will be a novel promising hole transporting material for highly efficient and stable PSCs.

Recent advances on aromatic-diimide based cathode interfacial materials for polymer solar cells

As an emerging photovoltaic technology, polymer solar cells (PSCs) have attracted broad interest for both academic and industry communities due to their outstanding advantages, such as lightweight, low cost and easy to process in large areas. In recent years, the power conversion efficiency of PSCs has been improved rapidly, exceeding 17% for the state-of-the-art devices. For device optimization, it has been demonstrated that the efficiency and stability can be significantly improved by constructing efficient organic cathode interfacial materials. Specially, aromatic-diimide based cathode interfacial materials have attracted much attention due to their advantages of excellent electron transport, easily chemical modification and suitable energy levels. In this review, along with the briefly introduction on the basic functions of cathode interlayer, recent progress of cathode interlayer based on perylene diimides and naphthalene diimides was summarized. At last, the perspective and challenge were proposed according to the requirements of practical application.

A chlorinated non-fullerene acceptor for efficient polymer solar cells

Improving the performance and reducing the manufacturing costs are the main directions for the development of organic solar cells in the future. Here, the strategy that uses chemical structure modification to optimize the photoelectric properties is reported. A new narrow bandgap (1.30 eV) chlorinated non-fullerene electron acceptor (Y15), based on benzo[d][1,2,3] triazole with two 3-undecyl-thieno[2′,3′:4,5] thieno[3,2-b] pyrrole fused -7-heterocyclic ring, with absorption edge extending to the near-infrared (NIR) region, namely A-DA'D-A type structure, is designed and synthesized. Its electrochemical and optoelectronic properties are systematically investigated. Benefitting from its NIR light harvesting, the fabricated photovoltaic devices based on Y15 deliver a high power conversion efficiency (PCE) of 14.13%, when blending with a wide bandgap polymer donor PM6. Our results show that the A-DA'D-A type molecular design and application of near-infrared electron acceptors have the potential to further improve the PCE of polymer solar cells (PSCs).

Conjugated Donor-Acceptor Terpolymers Toward High-Efficiency Polymer Solar Cells.

The development of conjugated alternating donor-acceptor (D-A) copolymers with various electron-rich and electron-deficient units in polymer backbones has boosted the power conversion efficiency (PCE) over 17% for polymer solar cells (PSCs) over the past two decades. However, further enhancements in PCEs for PSCs are still imperative to compensate their imperfect stability for fulfilling practical applications. Meanwhile development of these alternating D-A copolymers is highly demanding in creative design and syntheses of novel D and/or A monomers. In this regard, when being possible to adopt an existing monomer unit as a third component from its libraries, either a D' unit or an A' moiety, to the parent D-A type polymer backbones to afford conjugated D-A terpolymers, it will give a facile and cost-effective method to improve their light absorption and tune energy levels and also interchain packing synergistically. Moreover, the rationally controlled stoichiometry for these components in such terpolymers also provides access for further fine-tuning these factors, thus resulting in high-performance PSCs. Herein, based on their unique features, the recent progress of conjugated D-A terpolymers for efficient PSCs is reviewed and it is discussed how these factors influence their photovoltaic performance, for providing useful guidelines to design new terpolymers toward high-efficiency PSCs.

Over 13% Efficiency Ternary Nonfullerene Polymer Solar Cells with Tilted Up Absorption Edge by Incorporating a Medium Bandgap Acceptor

AbstractEfficient ternary polymer solar cells (PSCs) are prepared with poly‐[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′] dithiophene‐co‐3fluorothieno[3,4‐b]thiophene‐2‐carboxylate] (PTB7‐Th):COi8DFIC as host system and medium bandgap material BDTThIT‐4F as the third component. The power conversion efficiency of PSCs can be increased from 11.47% to 13.08% by incorporating 20 wt% BDTThIT‐4F in acceptors, along with the simultaneously improved three key photovoltaic parameters. The absorption edge of ternary blend films can be tilted up in long wavelength range by incorporating appropriate BDTThIT‐4F, although the bandgap of BDTThIT‐4F is wider than that of COi8DFIC, leading to the extended external quantum efficiency spectra of ternary PSCs. The tilted up absorption edge of blend films should be attributed to the variation of COi8DFIC molecular arrangement, which can be well demonstrated from the transient and steady absorption spectra of blend films with different donors and acceptors. A new ground state bleach signal can be clearly observed in transient absorption spectra of the optimized ternary blend films, which may be due to the varied COi8DFIC molecular energy levels by incorporating BDTThIT‐4F. Meanwhile, the lifetimes on excited states are increased in the ternary blend films, which is beneficial to exciton dissociation for improving the performance of ternary PSCs.

Weakening the Aggregations of Polymer Chains toward Efficient Non-Fullerene Polymer Solar Cells.

The 2D asymmetric benzodithiophene (BDT) unit is used as a donor unit to construct one new polymer PBDTBDD-Th with benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDD) as acceptor building block. In comparison to the polymer PBDTsTh-BDD with a side chain containing a sulfur atom, the devices based on PBDTBDD-Th/ITIC show better performance due to the introduction of carbon atoms in the side chain, which could weaken the self-aggregations of polymer chains. As a result, the devices based on PBDTBDD-Th/ITIC blends yield power conversion efficiencies (PCEs) over 10%, much higher than those based on PBDTsTh-BDD/ITIC blends (7.09%). The exciton dissociation probabilities (P diss ) of a device based on PBDTBDD-Th/ITIC blends is 95.3%, which suggests that the device achieves good exciton dissociation and charge transfer. In general, the polymer PBDTBDD-Th shows capability to increase the PCEs of polymer solar cells (PSCs) with a non-fullerene acceptor.

Polymer solar cells: P3HT:PCBM and beyond

For the last two decades, polymer solar cells (PSCs) have been a cynosure of the photovoltaic community, as evidenced by the growing number of patent applications and scientific publications. Efforts to achieve high power conversion efficiency in PSC, propelled by advances in device architecture, material combination, and nanomorphology control, evolved into poly(3-hexylthiophene-2,5-diyl) (P3HT):phenyl-C61-Butyric-Acid-Methyl Ester (PCBM) bulk heterojunction PSCs, which had been the best seller in PSC research for a decade. Subsequently, PSC research was redirected towards the synthesis of low bandgap materials and optimization of tandem cells, which led to a power conversion efficiency of ∼13%. Even though this efficiency may not be sufficient enough to compete with that of inorganic solar cells, unique properties of PSCs, such as mass roll-to-roll production capability, as well as flexibility and lightness, suggest their niche market opportunities. In this review, an overview of developments in PSCs is presented during the last three decades encompassing pre- and post-P3HT:PCBM era. Emphasis is given on evolution in device architecture, coupled with material selection for pre-P3HT:PCBM era, and synthesis of low-bandgap materials, coupled with a tandem structure for post-P3HT:PCBM era. Last but not least, efforts toward the longer operational lifetime of PSCs by encapsulation are reviewed.

NaYbF4:Tb/Eu modified with organic antenna for improving performance of polymer solar cells

Inorganic-organic nanocomposites are synthesized and applied in polymer solar cells (PSCs) for improving power conversion efficiency (PEC). The hexagonal NaYbF4:Tb/Eu nanocrystals are synthesized, and then two organic molecules, phenanthroline (Phen) and thenoyltrifluroacetone (TTA), are introduced to modify the surface of the NaYbF4:Tb/Eu. As organic antennas, Phen and TTA can absorb UV and then transfer energy to inorganic nanocrystals. The prepared nanocomposites combine upconversion (UC) and downshift (DS) process under excitation, which can convert them into visible light. Thus, the inorganic-organic nanocomposites display photoresponse both in UV and NIR region. Two organic ligands and Tb3+ play an indispensable role during the energy transfer process. The enhanced UC and tunable DS emission match well with absorption of active layer of PSC, which can enhance the performance of PSC. The open-circuit voltage (Voc) and short-circuit current density (Jsc) of the devices are about 0.77 V and 16.48 mA cm−2, respectively. And the PEC of PSC can be increased 7.85%, which is about 10.2% higher than that of the reference device without UC and DS.

Improving the performance of polymer solar cells by efficient optimizing the hole transport layer-graphene oxide

Graphene oxide (GO) materials have emerged as a promising alternative for hole transport layer (HTL) in polymer solar cells (PSCs) due to their unique structures and properties. However, insulating properties and eco-contaminative production of GO still need to be solved. Here, we report on the preparation of GO through an improved Hummers method without using NaNO3, which is an eco-friendly option because it avoids the emissions of NO2 and N2O4 toxic gases. Subsequently, the GO as HTL in PSCs is reduced by simple heat treatment of different temperatures in air, and the performance of devices is obviously improved. The FT-IR and XPS spectra show oxygenated functional groups in GO thin films are gradually removed with the increase of annealing temperature, which restores sp2 hybridized graphitic structure, and makes the GO thin films more conducive to the charge transfer. The highest power conversion efficiency of PSCs based on the P3HT: PC71BM system with GO as HTL is 3.39%, which approaches that of PSCs with PEDOT: PSS as HTL (3.41%). Moreover, the devices with annealed GO as HTL have better stability compared to devices with PEDOT: PSS.

Orienting the Microstructure Evolution of Copper Phthalocyanine as an Anode Interlayer in Inverted Polymer Solar Cells for High Performance.

Recent advances in the interfacial modification of inverted-type polymer solar cells (PSCs) have resulted from controlling the surface energy of the cathode-modified layer (TiO2 or ZnO) to enhance the short-circuit current (Jsc) or optimizing the contact morphology of the cathode (indium tin oxide or fluorine-doped tin oxide) and active layer to increase the fill factor. Herein, we report that the performance enhancement of PSCs is achieved by incorporating a donor macromolecule copper phthalocyanine (CuPc) as an anode modification layer. Using the approach based on orienting the microstructure evolution, uniformly dispersed island-shaped CuPc spot accumulations are built on the top of PTB7:PC71BM blend film, leading to an efficient spectral absorption and photogenerated exciton splitting. The best power conversion efficiency of PSCs is increased up to 9.726%. In addition to the enhanced light absorption, the tailored anode energy level alignment and optimized boundary morphology by incorporating the CuPc interlayer boost charge extraction efficiency and suppress the interfacial molecular recombination. These results demonstrate that surface morphology induction through molecular deposition is an effective method to improve the performance of PSCs, which reveals the potential implications of the interlayer between the organic active layer and the electrode buffer layer.

Fabrication Processes to Generate Concentration Gradients in Polymer Solar Cell Active Layers

Polymer solar cells (PSCs) are considered as one of the most promising low-cost alternatives for renewable energy production with devices now reaching power conversion efficiencies (PCEs) above the milestone value of 10%. These enhanced performances were achieved by developing new electron-donor (ED) and electron-acceptor (EA) materials as well as finding the adequate morphologies in either bulk heterojunction or sequentially deposited active layers. In particular, producing adequate vertical concentration gradients with higher concentrations of ED and EA close to the anode and cathode, respectively, results in an improved charge collection and consequently higher photovoltaic parameters such as the fill factor. In this review, we relate processes to generate active layers with ED–EA vertical concentration gradients. After summarizing the formation of such concentration gradients in single layer active layers through processes such as annealing or additives, we will verify that sequential deposition of multilayered active layers can be an efficient approach to remarkably increase the fill factor and PCE of PSCs. In fact, applying this challenging approach to fabricate inverted architecture PSCs has the potential to generate low-cost, high efficiency and stable devices, which may revolutionize worldwide energy demand and/or help develop next generation devices such as semi-transparent photovoltaic windows.

Processing temperature control of a diketopyrrolopyrrole-alt-thieno[2,3-b]thiophene polymer for high-mobility thin-film transistors and polymer solar cells with high open-circuit voltages

Abstract We synthesized a planar pDPPTTi-OD polymer based on diketopyrrolopyrrole (DPP) and thieno [2,3- b ]thiophene (TTi) and investigated the electrical properties of the pDPPTTi-OD polymer. pDPPTTi-OD films displayed a low optical bandgap of 1.57 eV, and HOMO and LUMO levels of −5.40 and −3.74 eV, respectively. The 150°C-annealed pDPPTTi-OD films showed a high hole mobility of 0.16 cm 2 V −1 s −1 in organic thin-film transistor (OTFT) devices. The photovoltaic properties of polymer solar cells (PSCs) incorporating the pDPPTTi-OD were also measured. A pDPPTTi-OD:PC 71 BM blend film was spin-coated at 25, 70 and 90 °C. High-temperature processing significantly improved the power conversion efficiency of PSCs by effectively reducing the PC 71 BM domain sizes, which improved the miscibility between pDPPTTi-OD and PC 71 BM. This work demonstrated that the TTi moiety is a useful donor building block for high- performance D–A type polymers in OTFTs and PSCs, and that processing temperatures should be controlled to fully realize the materials' beneficial intrinsic properties.

Improved Performance of Polymer Solar Cells by Thermal Evaporation of AgAl Alloy Nanostructures into the Hole-Transport Layer.

The performance characteristics of polymer solar cells (PSCs) incorporated with AgAl and Ag nanostructures and MoO3 spacer layers were investigated. The power conversion efficiency (PCE) of PSCs is sensitive to the nominal thicknesses of the AgAl nanostructures and the MoO3 spacer layer. The PCE of a PSC with a 3-nm-thick layer of AgAl nanostructures and a 1-nm-thick MoO3 isolation layer reached 9.79%, which is higher than the PCE (8.55%) of the reference PSC without metal nanostructures. Compared to PSCs with Ag nanostructures, PSCs with AgAl nanostructures showed better stability and still retained 60% of their initial PCE values after aging for 120 days in air without encapsulation. The enhanced stability of the PSCs is attributed to the formation of AlOx, which can inhibit the diffusion of Ag atoms into the neighboring layer. The localized surface plasmonic resonance (LSPR) effect of AgAl nanostructures was retained by inserting an only 1-nm-thick MoO3 spacer layer between the metal nanostructures and the metal electrode. Our work has demonstrated that using AgAl alloy instead of Ag as plasmonic nanostructures is a better strategy for improving the performance of PSCs, especially in terms of the stability of the cells.

Luminescent GdVO4:Sm3+ quantum dots enhance power conversion efficiency of bulk heterojunction polymer solar cells by Förster resonance energy transfer

In this work, we report enhanced power conversion efficiency (PCE) of bulk heterojunction polymer solar cells by Förster resonance energy transfer (FRET) from samarium-doped luminescent gadolinium orthovanadate (GdVO4:Sm3+) quantum dots (QDs) to polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) polymer. The photoluminescence emission spectrum of GdVO4:Sm3+ QDs overlaps with the absorption spectrum of PTB7, leading to FRET from GdVO4:Sm3+ to PTB7, and significant enhancements in the charge-carrier density of excited and polaronic states of PTB7 are observed. This was confirmed by means of femtosecond transient absorption spectroscopy. The FRET from GdVO4:Sm3+ QDs to PTB7 led to a remarkable increase in the power conversion efficiency (PCE) of PTB7:GdVO4:Sm3+:PC71BM ([6,6]-phenyl-C71-butyric acid methyl ester) polymer solar cells. The PCE in optimized ternary blend PTB7:GdVO4:Sm3+:PC71BM (1:0.1:1.5) is increased to 8.8% from 7.2% in PTB7:PC71BM. This work demonstrates the potential of rare-earth based luminescent QDs in enhancing the PCE of polymer solar cells.

Low-temperature preparation of a molybdenum oxide hole collection layer by using a peroxo precursor for polymer solar cells

Amorphous molybdenum oxide (MoOx) as a hole collection layer (HCL) in polymer solar cells (PSCs) was prepared by annealing of a peroxo precursor. The power conversion efficiency of PSCs based on a blend of poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester was improved to 3.19% by using an MoOx layer prepared by annealing at 200°C in N2 as an alternative for poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). X-ray photoelectron spectra of MoOx layers showed that the peroxide precursor was converted into MoOx by annealing at 100–150°C and that annealing at a high temperature in N2 under unaerated conditions promotes the reduction of Mo6+ species into Mo5+ species. In contrast, ultraviolet photoelectron spectra of an MoOx layer showed that the electric structure of MoOx depends on not the amount of Mo5+ species but the presence or absence of Mo5+ species. These results indicated that an appropriate amount of Mo5+ species in the MoOx layer is important for an efficient MoOx HCL. A stability test revealed the PSCs with MoOx layers had relatively high device stability in air compared to that of PSCs with PEDOT:PSS.

D–A copolymer with high ambipolar mobilities based on dithienothiophene and diketopyrrolopyrrole for polymer solar cells and organic field-effect transistors

Donor–acceptor (D–A) type conjugated polymers have been developed to absorb longer wavelength light in polymer solar cells (PSCs) and to achieve a high charge carrier mobility in organic field-effect transistors (OFETs). PDTDP, containing dithienothiophene (DTT) as the electron donor and diketopyrrolopyrrole (DPP) as the electron acceptor, was synthesized by stille polycondensation in order to achieve the advantages of D–A type conjugated polymers. The polymer showed optical band gaps of 1.44 and 1.42eV in solution and in film, respectively, and a HOMO level of 5.09eV. PDTDP and PC71BM blends with 1,8-diiodooctane (DIO) exhibited improved performance in PSCs with a power conversion efficiency (PCE) of 4.45% under AM 1.5G irradiation. By investigating transmission electron microscopy (TEM), atomic force microscopy (AFM), and the light intensity dependence of JSC and VOC, we conclude that DIO acts as a processing additive that helps to form a nanoscale phase separation between donor and acceptor, resulting in an enhancement of μh and μe, which affects the JSC, EQE, and PCE of PSCs. The charge carrier mobilities of PDTDP in OFETs were also investigated at various annealing temperatures and the polymer exhibited the highest hole and electron mobilities of 2.53cm2V−1s−1 at 250°C and 0.36cm2V−1s−1 at 310°C, respectively. XRD and AFM results demonstrated that the thermal annealing temperature had a critical effect on the changes in the crystallinity and morphology of the polymer. The low-voltage device was fabricated using high-k dielectric, P(VDF-TrFE) and P(VDF-TrFE-CTFE), and the carrier mobility of PDTDP was reached 0.1cm2V−1s−1 at Vd=−5V. PDTDP complementary inverters were fabricated, and the high ambipolar characteristics of the polymer resulted in an output voltage gain of more than 25.

Performance improvement of inverted polymer solar cells thermally evaporating CuI as an anode buffer layer

The p-type semiconductor copper iodide (CuI) was incorporated into P3HT:ICBA bulk heterojunction polymer solar cells (PSCs) as a novel anode buffer layer. The power conversion efficiency of PSCs with 2nm CuI is 3.84%, about 1.6 times higher than that of the reference device without CuI, which is due to the simultaneous enhanced photocurrent (Jsc), open current voltage (Voc) and fill factor (FF). The CuI buffer layer can module the Schottky barrier and form an ohmic contact at the P3HT:ICBA/Au interface, which act as both holes transport layer and electron block layer. The complex impedance spectra indicate that CuI buffer layer can also effectively decrease the series resistance of whole device.

Effect of electrode geometry on photovoltaic performance of polymer solar cells

This paper investigates the impact of electrode geometry on the performance of polymer solar cells (PSCs). The negative electrodes with equal area (0.09 cm2) but different shape (round, oval, square and triangular) are evaluated with respect to short-circuit current density, open-circuit voltage, fill factor and power conversion efficiency of PSCs. The results show that the device with round electrodes gives the best photovoltaic performance; in contrast, the device with triangular electrodes reveals the worst properties. A maximum of almost a 19% increase in power conversion efficiency with a round electrode is obtained in the devices compared with that of the triangular electrode. To conclude, the electrode boundary curvature has a significant impact on the performance of PSCs. The larger curvature, i.e. sharper electrodes edges, perhaps has a negative effect on exciton separation and carrier transport in photoelectric conversion processes.