Properties Of Polymer Solar Cells Research Articles

Tailoring Self‐Assembled Monolayers for High‐Performance Polymer Solar Cells with Improved Stability

Self‐assembly monolayers (SAMs) of small molecules are attractive alternatives of traditional transporting materials to reconcile interfaces with tunable interface properties in polymer solar cells (PSCs). Herein, it is found that benzylphosphonic acid (BnPA)/pentafluorobenzylphosphonic acid (F5BnPA) mixture could form an ordered SAM on indium tin oxide (ITO) due to the strong arene–perfluoroarene interaction, thus the hole‐transport‐layer‐free PSCs based on poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl‐3‐fluoro)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]‐dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione)] (PM6): BO‐4Cl achieves a power conversion efficiency (PCE) of 18.0%. The high performance is attributed to the improved energy level alignment, excellent carrier‐extraction ability, and reduced recombination. The device also shows much better stability compared with the devices based on BnPA‐ or F5BnPA‐modified ITO, and shows comparable stability to the device based on ITO/poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Furthermore, the device with an area of 1.05 cm2 shows a PCE of 15.3%, which is among the highest reported values. Herein, the potential of SAMs is highlighted for highly stable and high‐performance PSCs toward commercialization.

Effect of UV-Light Irradiation on Charge-Accumulation States in PTzBT Polymer Solar Cells

Ternary polymer solar cells based on a polymer PTzBT have received a lot of attention because their power conversion efficiency (PCE) and thermal stability have been greatly improved by adding a small amount of an oligomer ITIC. However, the charge-accumulation states of the PTzBT ternary polymer solar cells have not yet been completely clarified. Here, we report electron spin resonance (ESR) spectroscopy of the PTzBT ternary polymer solar cells to investigate the charge-accumulation states from a microscopic viewpoint with examining the effect of UV-light irradiation. We observed a slight increase in the ESR intensity of the PTzBT cells and layered film samples under simulated solar irradiation with a UV-cut filter. Multiple signals were observed in the PTzBT cells under solar irradiation when the UV-cut filter was removed. Interestingly, no decrease was observed in the ESR signal of ZnO under solar irradiation with the UV-cut filter, although holes in ZnO in the PTzBT cells are known to decrease under solar irradiation. These findings will contribute to a deep understanding of the material properties of polymer solar cells.

INFLUENCE OF STRUCTURAL FEATURES OF ZNO FILMS ON OPTICAL AND PHOTOELECTRIC CHARACTERISTICS OF INVERTED POLYMER SOLAR ELEMENTS

In this work we investigated the effect of preliminary annealing of zinc acetate solution films on the morphology, structure, optical properties of the formed ZnO films and also on the photovoltaic properties of polymer solar cells based on the obtained ZnO films. It was found that the pre-annealing temperature significantly affects the morphology and structure of the obtained ZnO films. At pre-annealing temperatures below 200 oC the films have a strongly relief morphology (wrinkled morphology), while at pre-annealing temperatures above 200 oC the surface morphology of the films is smooth. The relief of ZnO films affects the photocurrent density of solar cells. Cells based on ZnO films with wrinkled morphology showed a higher photocurrent compared to smooth morphology, which is due to strong light scattering and, as a result, the optical path of light in the photoactive layer is increased due to multiple reflection of light in the wrinkled structure of ZnO. In addition, with increasing pre-annealing temperature, the photovoltage of solar cells and the rate of recombination of charge carriers increases, but the diffusion coefficient of charge carriers decreases, which indicates an increase in the density of defects in the crystal lattice of ZnO. Thus, it has been shown that smooth or highly relief thin ZnO films with controlled properties can be obtained from a zinc acetate solution.

Ternary solar cells via ternary polymer donors and third component PC71BM to optimize morphology with 13.15% efficiency

The third component played an increasingly significant role in polymer solar cells (PSCs), especially for the morphological impact. Here, we use a ternary polymer donor (TBFCl50-BDD) and organic small molecule acceptor (BTP-4F) as the main material and introduce a third component (PC71BM) to enhance the morphology of the photoactive layer and result in improved device performance. The TBFCl50-BDD:BTP-4F based PSCs only exhibited an efficiency of 10.58%, while The power conversion efficiency of ternary device (TBFCl50-BDD:BTP-4F:PC71BM) reached 13.15%. Importantly, the introduction of PC71BM can not only improve the morphology of the active layer and promote more efficient exciton transport, but also influence the molecular arrangement for better crystallization performance. This work offers an effective method for optimization of the morphology to enhance the properties of PSCs.

High-Molecular-Weight Electroactive Polymer Additives for Simultaneous Enhancement of Photovoltaic Efficiency and Mechanical Robustness in High-Performance Polymer Solar Cells.

The development of small-molecule acceptors (SMAs) has significantly enhanced the power conversion efficiency (PCE) of polymer solar cells (PSCs); however, the inferior mechanical properties of SMA-based PSCs often limit their long-term stability and application in wearable power generators. Herein, we demonstrate a simple and effective strategy for enhancing the mechanical robustness and PCE of PSCs by incorporating a high-molecular-weight (MW) polymer acceptor (PA, P(NDI2OD-T2)). The addition of 10–20 wt % PA leads to a more than 4-fold increase in the mechanical ductility of the SMA-based PSCs in terms of the crack onset strain (COS). At the same time, the incorporation of PA into the active layer improves the charge transport and recombination properties, increasing the PCE of the PSC from 14.6 to 15.4%. The added PAs act as tie molecules, providing mechanical and electrical bridges between adjacent domains of SMAs. Thus, for the first time, we produce highly efficient and mechanically robust PSCs with a 15% PCE and 10% COS at the same time, thereby demonstrating their great potential as stretchable or wearable power generators. To understand the origin of the dual enhancements realized by PA, we investigate the influence of the PA content on electrical, structural, and morphological properties of the PSCs.

Fluorene을 기본골격으로 한 공액형 고분자 전해질을 Interlayer로 적용한 고분자태양전지의 광전효과

초록·키워드 목차 오류제보하기 고분자의 주사슬 구조가 광전 특성에 어떠한 영향을 미치는지에 관하여 조사하기 위해 고분자 전해질, 6,6"-(2-phenyl-9H-fluorene-9,9-diyl)bis(N,N,N-trimethylhexan-1-aminium) bromide(PFB-Br), 6,6"-(2-(thiophen-2-yl)-9Hfluorene-9,9-diyl)bis(N,N,N-trimethylhexan-1-aminium) bromide(PFT-Br), 6,6"-(2-([2,2"-bithiophen]-5-yl)-9H-fluorene-9,9-diyl)bis(N,N,N-trimethylhexan-1-aminium) bromide(PF2T-Br)을 합성하여 유기 태양광 소자의 중간층으로 도입하였다. 이온성 곁사슬은 스핀 코팅 과정 도중에 ZnO와 상호 작용으로 인해 ZnO 표면으로 자발적으로 배향되며, 주사슬은 반대 방향으로 재배열된다. 고분자 전해질의 재배열로 인해 소자에서 일정한 배향을 가지는 영구 쌍극자가 형성됨으로써, 추가적인 내부 자기장이 형성된다. 주사슬에 도입된 벤젠에 비해 높은 전자 밀도를 가지는 thiophene과 bithiophene에 의해 서로 다른 전자친화도를 가지게 되어 PFT-Br, PF2T-Br의 전자 전달이 방해되고 PFB-Br에 비하여 더 낮은 전력 변환 효율을 나타냄을 확인하였다. #polymer solar cell #fluorene-based polyelectrolyte #thiophene #cathode interlayer 초록AbstractIntroductionResults and DiscussionConclusionsReferences

Impact of Chalcogenophenes on Donor-Acceptor Copolymers for Bulk Heterojunction Solar Cells

Three new selenophene-based conjugated copolymers having different ratios of the monomeric units were designed, synthesized and thoroughly characterized. The introduction of an electron-poor and surfaced building moiety like selenathiazole was highly efficient in tuning the bandgap and polymer properties. The chalcogenophene-based medium-bandgap polymers demonstrated low-lying HOMO energy levels (∼5.87 eV), which is benign for use in multi-junction polymer solar cell applications. The representative polymers with heavy atoms revealed the change in electronegativity and atomic size that highly affected the molecular property, its topological features, and photovoltaic properties in polymer solar cells. The selenium-substituted (0.5:0.5) polymer donors showed power conversion efficiencies above 3% when combined with [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) acceptors in a quintessential bulk-heterojunction solar cell.

The influence of structural and charge transport properties of PEDOT:PSS layers on the photovoltaic properties of polymer solar cells

In this study, the effect of modification of poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) morphology on its optical, charge transport, and photovoltaic properties of organic solar cells based on the P3HT:PC60BM bulk heterojunction is presented. It is shown that the addition of isopropyl alcohol to the PEDOT:PSS polymer solution and annealing of spin‐coated PEDOT:PSS film leads to a change in its morphology, charge transport, and optical properties. By an impedance spectroscopy technique, the charge transport properties of PEDOT:PSS films were studied. It was established that the efficiency of carrier transport and the efficiency of the polymer solar cells depends on the structural features of PEDOT:PSS.

Orthogonal Printable Reduced Graphene Oxide 2D Materials as Hole Transport Layers for High-Performance Inverted Polymer Solar Cells: Sheet Size Effect on Photovoltaic Properties.

Creating an orthogonal printable hole-transporting layer (HTL) without damaging the underlying layer is still a major challenge in fabricating large-area printed inverted polymer solar cells (PSCs). In this study, we prepared orthogonal-processable fluorine-functionalized reduced graphene oxide (FrGO) series with various two-dimensional sheet sizes such as large-sized FrGO (1.1 μm), medium-sized FrGO (0.7 μm), and small-sized FrGO (0.3 μm) and systematically investigated the size effect of FrGOs on the hole transport properties of PSCs. The FrGOs exhibit highly stable dispersion without change over 90 days in 2-propanol solvent, indicating very high dispersion stability. Decreasing the sheet size of FrGOs enhanced hole-transporting properties, resulting in power conversion efficiencies (PCEs) of 9.27 and 9.02% for PTB7-Th:EH-IDTBR- and PTB7-Th:PC71BM-based PSCs, respectively. Compared to devices with solution-processed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), a 14% enhancement of PCEs was achieved. Interestingly, the PCEs of devices with the smallest FrGO sheet are higher than the PCE of 8.77% of a device with vacuum-deposited MoO3. The enhancement in the performance of PSCs is attributed to the enhanced charge collection efficiency, decreased leakage current, internal resistance, and minimized charge recombination. Finally, small-sized FrGO HTLs were successfully coated on the photoactive layer using the spray coating method, and they also exhibited PCEs of 9.22 and 13.26% for PTB7-Th:EH-IDTBR- and PM6:Y6-based inverted PSCs, respectively.

New Conjugated Polymers Based on Dithieno[2,3‐e:3′,2′‐g]Isoindole‐7,9(8H)‐Dione Derivatives for Applications in Nonfullerene Polymer Solar Cells

The fast evolution of the narrow bandgap nonfullerene acceptors requires new conjugated wide bandgap polymers for the use of nonfullerene polymer solar cells (PSCs). Herein, two new wide bandgap A1–D1–A2–D1 conjugated polymers with the same dithieno[2,3‐e:3′,2′‐g]isoindole‐7,9(8H)‐dione (DTID) acceptor (A1) and thiophene donor (D1) and different A2 acceptor units, i.e., benzothiadiazole (BT) and fluorinated benzothiadiazole (f‐BT) denoted as P113 and P114, are designed, and the effect of fluorination of the BT acceptor unit on the photovoltaic properties of PSCs using nonfullerene acceptors is investigated. It is found that the incorporation of fluorine atom into the BT acceptor unit increases the absorption coefficients and the relative dielectric constant. The increase in photoluminescence quenching, reduction in charge recombination loss, and improvement in the charge carrier life are observed for P114. All these factors result in the drastically improved power conversion efficiency of P114:ITIC‐m‐based PSC to 10.42% with a small energy loss of 0.56 eV as compared with the P113 counterpart (8.74% with an energy loss of 0.69 eV) under identical conditions. The low energy loss is beneficial to overcome the trade‐off between open‐circuit voltage and short‐circuit current.

Influence of Acceptor Type and Polymer Molecular Weight on the Mechanical Properties of Polymer Solar Cells

Nonfullerene acceptors (NFAs), that are small-molecule acceptors (SMA) and polymer acceptors (PAs), have been extensively explored, which has yielded significant enhancements in the photovoltaic performance of polymer solar cells (PSCs). The mechanical robustness of the PSCs is of vital and equal importance to ensure long-term stability and enable their use as power-generators in flexible and stretchable electronics. Here, we report a comparative study of the mechanical properties of SMA-based, PA-based, and fullerene-based PSCs. We chose ITIC (SMA), P(NDI2OD-T2) (PA), and PCBM (fullerene) as three representative acceptor materials and blended them with the same polymer donor PTB7-Th. To understand the difference between the mechanical properties of SMA-based and PA-based PSCs, we control the number-average molecular weight (Mn) of P(NDI2OD-T2) from 15 to 163 kg mol–1 in all-PSCs. The all-PSCs-based high-Mn PAs exhibit significantly higher cohesion energy (4.03 J m–2) than SMA-PSCs (1.19 J m–2) and PCBM-PSCs (0.29 J m–2). Notably, the all-PSCs exhibit a high strain at fracture of 31.1%, which is 9- and 28-fold higher than those of SMA-PSCs and PCBM-PSCs, respectively. The superior mechanical robustness of all-PSCs is attributed to using a PA above the critical molecular weight (Mc), which produces tie molecules and polymer entanglements that dissipate substantial mechanical strain energy with large plastic deformation. This work provides useful design guidelines for photovoltaic active materials in terms of the mechanical properties and highlights the importance of incorporating high-Mn PAs above the Mc for producing PSCs with excellent mechanical robustness and device performance.

Effect of interface modification in polymer solar cells: An in-depth investigation of the structural variation of organic dye for interlayer material

In order to investigate the effect of the structural variation of interlayer materials on the photovoltaic properties of polymer solar cells (PSCs) in depth, we designed and synthesized three types of small-molecule dyes with a structure of 1,1′-bis(1-alkyl)-4,4′-bipyridine-1,1′-diium benzenesulfonate (V-alkyl-OTs). Here, the alkyl groups were butyl, hexyl, and dodecyl, which denoted as C4, C6, and C12, respectively. The magnitudes dipole moments will be in the order of V–C4-OTs < V–C6-OTs < V–C12-OTs due to the increased the alkyl chain length from C4 to C12. The work function of the ZnO layer with V-alkyl-OTs is exhibited to depend on the alkyl chain length, indicating that a Schottky barrier can be tuned by the size of cation part. Thus, The power conversion efficiencies (PCEs) of the PSCs based on the blend of PTB7 and PC71BM as the photoactive layer with V-alkyl-OTs were improved over for the device with pristine ZnO (without V-Alkyl-OTs) from 7.6% (short circuit current (Jsc) = 16.0 mA/cm2, open circuit voltage (Voc) = 0.72 V, fill factor (FF) = 65.6%) to 8.1% (V–C4-OTs, Jsc = 16.8 mA/cm2, Voc = 0.73 V, FF = 65.9%), 8.3% (V–C6-OTs, Jsc = 17.2 mA/cm2, Voc = 0.72 V, FF = 67.3%), and 8.6% (V–C12-OTs, Jsc = 18.0 mA/cm2, Voc = 0.72 V, FF = 66.4%). The enhancement of the PCE is strongly related to the alkyl chain length, and the major contribution was by the improvement of the Jsc due to the reduction in the energy offset at the cathode interface.

Photoinduced Slowdown of Charge Carrier Transport and Phonon Emission in Conjugated Polymers

In a real bulk heterojunction polymer solar cell, once the photoactive polymer layer undergoes photoexcitation, the formed electron–hole pair (or exciton) diffuses to the fabricated heterojunction to be dissociated, and then the resultant charge carriers move toward the cathode and anode. During the negative polaron transport, the external light field excites the polaron and makes it transport slowly where the excitation selects three localized phonon modes with even parity and higher frequency, not only greatly deepening the local distortion along alternating bonds but also leading to blue shift in the infrared phonon spectrum. It is furthermore discovered that the slowdown of the charged polaron’s velocity is contributed by the occurrence of the phonon modes and the blue shift of the phonon spectrum. Controlling the phonon modes and the frequency blue shift of the phonon spectrum in a composite conjugated polymer solar cell presents an opportunity for modifying the properties of polymer solar cells.

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.

The investigations of two conjugated polymers that show distinctly different photovoltaic properties in polymer solar cells

In fullerene-free polymer solar cells (PSC), the non-fullerene small molecular acceptors (NF-SM) demonstrates quite different film morphology in comparison with the conventional fullerene acceptors when blended with the same conjugated polymer donor. In this work, a NF-SM acceptor, ITIC, was introduced into two efficient binary PSCs based on fullerene to fabricate ternary solar cells. It is found that the addition of ITIC led fundamentally different results. After carefully investigated the difference on film morphology and charge carriers' mobility, the results showed that the good miscibility between ITIC and the polymer may deteriorate the favorable film morphology with appropriate phase separation and suppress the formation of continues charge transport channel. This work offered a useful consideration to produce high-performance ternary PSCs by rational selecting the donors and acceptors.

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.

Highly efficient two-dimensional conjugated benzodithiophene-based photovoltaic polymers

In order to improve the photovoltaic properties of polymer solar cells (PSC), varieties of donor and acceptor units were designed and synthesized to construct the highly efficient conjugated polymers. Recent years, materials based on benzo [1.2-b:4,5-b ¢ ]dithiophene (BDT) units have showed promising applications in the field of organic photovoltaics for its excellent properties. As compared to BDT-based polymers with flexible side chains, polymers based on two-dimensional conjugated BDT (2D-BDT) usually had better thermal stability, slightly red-shifted of absorption spectra, lower HOMO levels, higher hole mobility and consequently improved photovoltaic properties. By using novel 2D-BDT-based polymers, the power conversion efficiency (PCE) of the PSC devices has been boosted to over 10%. In this article, we first briefly introduce the synthesis methods of 2D-BDT unit, and then focus our attention to summarize the numerous of 2D-BDT-based polymers and their applications in highly efficient PSC devices.

Polymer solar cells with enhanced power conversion efficiency using nanomaterials and laser techniques

Organic solar cells are photovoltaic cells and a type of green energy source with great potential application due to low production costs and mechanical flexibility. This study focused on the study of the electrical, optical and structure properties of polymer solar cells (PSCs) with many layers using newly synthesised polyimine. The molecular structure of the polyimine was characterised by Fourier transformation infrared and nuclear magnetic resonance spectroscopies as well as thermogravimetric analysis. The incorporated Al2O3 and TiO2 nanoparticles (NPs) in the active layer of the PSCs increased the absorbance of the polymer. Solar cells were constructed with a suitable coating and good efficiency using a spin-coating technique, and the efficiency was enhanced using a laser. The efficiencies of the PSCs that contain Al2O3 NPs before using a laser at 3.7% and after using a laser at 5.6% as well as the PSCs that contain TiO2 before using a laser at 10.052 and 12.3% were investigated. Importantly, this study provides a new approach for enhancing the performance of PSCs using nanomaterials and laser techniques

The effect of meta-substituted or para-substituted phenyl as side chains on the performance of polymer solar cells

Two donor-acceptor alternative copolymers (P1 and P2) with carbazole group as the donor unit and 2,3-diphenylquinoxaline group as the acceptor unit were designed and synthesized by Suzuki polycondensation reaction. Alkoxy groups were linked to meta-position or para-position of phenyl rings, which were attached on the 2,3-position of quinoxaline group to investigate the effect of the alkoxy position at phenyl rings on the resulting polymer optical, electrochemical and photovoltaic properties. GIWAXS experiments revealed that para-substituted polymer P2 possesses closer packing properties in the solid state. The absorption spectra of para-substituted polymer P2 also displayed larger red-shift than that of meta-substituted polymer P1 when it went from solution to the film, which was beneficial to improve the hole mobilities and short circuit current density (Jsc). When used as the donor materials, devices based on P2:PC71BM exhibited a PCE of 3.33%, which was obviously higher than that based on P1:PC71BM. Our results revealed that the slightly structure change of polymer has a significant influence on the photovoltaic properties of polymer solar cells and adopting para-substituted phenyl ring as the side groups gave superior photovoltaic performance in donor materials based on carbazole and quinoxaline.

A Silica Colloidal Mask for Randomly Surface‐textured Indium‐doped Tin Oxide for Enhancing the Power‐conversion Efficiency of Polymer Solar Cells

We report on the photovoltaic properties of polymer solar cells (PSCs) that utilize surface‐textured indium‐doped tin oxide (ITO) as the hole‐collecting electrode. The surface‐textured ITO was obtained by etching the ITO surface using two‐dimensional silica colloidal particles. By treating the ITO surface with polyelectrolytes, monolayer silica colloidal particles could be uniformly distributed on the ITO surface. The random patterns on the ITO surface were realized by a plasma etching process using these colloidal particles as an etching mask. Scattered reflection induced by the random patterns on the ITO surface leads to increased photon absorption in the photoactive layer. The PSC with an ITO etching depth of 30 nm exhibits an increased short‐circuit current density of 10.0 mA/cm2 and a power‐conversion efficiency of 4.23%, compared to the reference device comprising an unpatterned ITO surface.