Large-area Polymer Solar Cells Research Articles

Chlorinated Fused Nonacyclic Non-Fullerene Acceptor Enables Efficient Large-Area Polymer Solar Cells with High Scalability

As the power conversion efficiency (PCE) of polymer solar cells (PSCs) keeps increasing, developing high-performance large-area PSCs toward commercial application becomes a hot topic in this field. Here, we design and synthesize a non-fullerene acceptor TfIF-4Cl with a fused nonacyclic core via end-group chlorination. Compared to the fluorinated counterpart (TfIF-4F), the combination of the Cl atom in TfIF-4Cl not only leads to red-shifted absorption, but also improves the molecular packing ability. When TfIF-4Cl is blended with the polymer donor PM7 to fabricate the PSC, the device maintains high VOC (0.97 V, corresponding to a low energy loss of 0.60 eV) and presents enhanced JSC and high fill factor (78%), thus leading to an improved efficiency of 14% when compared with a PM7:TfIF-4F-based device. The high VOC but moderate photocurrent of PM7:TfIF-4Cl-based small-area device are beneficial to achieve large-area devices with high scalability according to the Joule’s law; thus, we manufacture 1.01 cm2 devices and achieve a remarkable PCE of 13.3%, which retain 95% in efficiency when the device area is scaled up from 0.04 to 1.01 cm2, and represent the best scalability when the device area is scaled up to 1 cm2 in the literature.

Efficient organic-inorganic hybrid cathode interfacial layer enabled by polymeric dopant and its application in large-area polymer solar cells

An organic-inorganic hybrid cathode interfacial layer (CIL) was developed by doping ZnO with the naphthalene-diimide based derivative NDI-PFNBr. It was found the resulting organic-inorganic hybrid CIL showed apparently improved conductivity and could act as an effective cathode interlayer to modify indium tin oxide (ITO) transparent electrodes. As a result, by employing the blend of PTB7-Th:PC71BM as the photoactive layer, the inverted polymer solar cells (PSCs) exhibited a remarkable enhancement of power conversion efficiency (PCE) from 8.52% for the control device to 10.04% for the device fabricated with the hybrid CIL. Moreover, all device parameters were simultaneously improved by using this hybrid CIL. The improved open-circuit voltage ( V OC) was attributed to the reduced work function of the ITO cathode, whereas the enhancements in fill factor (FF) and short-circuit current density ( J SC) were assigned to the increased conductivity and more effective charge extraction and collection at interface. Encouragingly, when the thickness of the hybrid CIL was increased to 80 nm, the resulting device could still keep a PCE of 8.81%, exhibiting less thickness dependence. Considering these advantages, 16 and 93 cm2 large-area PSCs modules were successfully fabricated from the hybrid CIL by using doctor-blade coating techniques and yielded a remarkable PCE of 8.05% and 4.49%, respectively. These results indicated that the hybrid CIL could be a promising candidate to serve as the cathode interlayer for high-performance large-area inverted PSCs.

Edge-functionalized graphene quantum dots as a thickness-insensitive cathode interlayer for polymer solar cells

A thickness-insensitive cathode interlayer (CIL) is necessary for large-area polymer solar cells (PSCs), in which thickness variation is unavoidable. These CIL materials are typically based on n-type conjugated polymer/molecule backbones, which show strong light absorption in the visible/near-infrared (NIR) region. This interferes with the sunlight absorption by the active layer and deteriorates device efficiency. In this study, we developed graphene quantum dots functionalized with ammonium iodide (GQD-NI) at the edge as a thickness-insensitive CIL with high optical transparency. The peripheral ammonium iodide groups of GQD-NI formed the desired interfacial dipole with the cathode to decrease the work function. The graphene basal planes of GQD-NI with a lateral size of ca. 3 nm demonstrated a good conductivity of 3.56 × 10–6 S·cm–1 and high transparency in the visible/NIR region (λmaxabs = 228 nm). Moreover, GQD-NI was readily soluble in polar organic solvents, e.g., methanol, which enabled multilayer device fabrication with orthogonal solvent processing. As a result, the PSC device with GQD-NI as the CIL exhibited a power conversion efficiency (PCE) of 7.49%, which was much higher than that of the device without the CIL (PCE = 5.38%) or with calcium as the CIL (PCE = 6.72%). Moreover, the PSC device performance of GQD-NI was insensitive to the GQD-NI layer thickness in the range of 2–22 nm. These results indicate that GQD-NI is a very promising material for application as a CIL in large-area printed PSCs.

Toward Enhancing Solar Cell Performance: An Effective and "Green" Additive.

Performance of bulk heterojunction polymer solar cells (PSCs) highly relies on the morphology of the photoactive layer involving conjugated polymers and fullerene derivatives as donors and acceptors, respectively. Herein, butylamine was found to be able to optimize the morphology of the donor/acceptor (D/A) film composed of a blend of poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). Compared to the commonly used alkane dithiols and halogenated additives with high boiling points, butylamine has a much lower boiling point between 77 and 79 °C, and it is also much "greener". A specific interaction between butylamine and PCBM was demonstrated to account for the morphology improvement. Essentially, butylamine can selectively dissolve PCBM in the P3HT:PCBM blend and facilitate the diffusion of PCBM in the film fabrication processes. Atomic force microscopy and X-ray photoelectron spectroscopy investigations confirmed the formation of the P3HT-enriched top surface and the abundance of PCBM at the bottom side, i.e., the formation of vertical phase segregation, as a consequence of the specific PCBM-butylamine interaction. The D/A film with inhomogeneously distributed D and A components in the vertical film direction, with more P3HT at the hole extraction side and more PCBM at the electron extraction side, enables more efficient charge extraction in the D/A film, reflected by the largely enhanced fill factor. The power conversion efficiency of devices reached 4.03 and 4.61%, respectively, depending on the thickness of the D/A film, and these are among the best values reported for P3HT:PCBM-based devices. As compared to the devices fabricated without the introduction of butylamine under otherwise the same processing conditions, they represented 19.6 and 21.6% improvement in the efficiency, respectively. The discovery of butylamine as a new, effective additive in enhancing the performance of PSCs strongly suggests that the differential affinity of additives toward donors and acceptors likely plays a more important role in morphology optimization than their boiling point, different from what was reported previously. The finding provides useful information for realizing large-area PSC fabrication, where a "greener" additive is always preferred.

Alkali Salt-Doped Highly Transparent and Thickness-Insensitive Electron-Transport Layer for High-Performance Polymer Solar Cell.

Solution-processable highly transparent and thickness-insensitive hybrid electron-transport layer (ETL) with enhanced electron-extraction and electron-transport properties for high-performance polymer solar cell was reported. With the incorporation of Cs2CO3 into the poly[(9,9-bis(6'-((N,N-diethyl)-N-ethylammonium)-hexyl)-2,7-fluorene)-alt-1,4-diphenylsulfide]dibromide (PF6NPSBr) ETL, the power conversion efficiency (PCE) of resulted polymer solar cells (PSCs) was significantly enhanced due to the favorable interfacial contact, energy-level alignment, and thus facile electron transport in the PSC device. These organic-inorganic hybrid ETLs also exhibited high transparency and high electron mobility. All of these combined properties ensured us to design novel thickness-insensitive ETLs that avoid the parasitic absorption of ETL itself simultaneously. With the conventional device structure with poly{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl} (PTB7-Th) as a donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor, devices with hybrid ETLs exhibited PCE of 8.30-9.45% within a wide range of ETL thickness. A notable PCE of 10.78% was achieved with the thick active layer poly(2,5-thiophene-alt-5,5'-(5,10-bis(4-(2-octyldodecyl)thiophen-2-yl)naphtho[1,2-c:5,6-c']bis([1,2,5]thiadiazole)) (PTNT812):PC71BM. These findings indicated that doping alkali salt into the organic interfacial materials can be a promising strategy to design highly efficient and thickness-insensitive ETL, which may be suitable for large-area PSC modules device fabrication with roll-to-roll printing technique.

Fullerene based electron transporting layers for large area polymer solar cells

ABSTRACTAn efficient fullerene based electron transporting material, C60,N,N,N -trimethyl-1-(2,3,4-tris(2-(2-methoxyethoxy)ethoxy)phenyl)dimetha-naminium monoadduct iodide salt (LT-S9079), is introduced to large area flexible polymer solar cells (PSCs). The excellent conductivity and wettability of the LT-S9079 on the active layer enable the fabrication of large-area slot-die coated PSCs of conventional photovoltaic device structure. The PSCs incorporating LT-S9079 demonstrate high current densities of 8.14 mA cm−2 and power conversion efficiencies of 1.46%.

Fluorination as an effective tool to increase the photovoltaic performance of indacenodithiophene-alt-quinoxaline based wide-bandgap copolymers

To investigate the effect of the fluoride phenyl side-chains into quinoxaline (PQx) unit on the photovoltaic performances of polymers, we demonstrated the synthesis and characterization of two novel wide-bandgap (WBG) copolymers, PIDT-DTPQx and PIDT-DTFPQx, in which indacenodithiophene (IDT), 2,3-diphenylquinoxaline (PQx) (and/or 2,3-bis(4-fluorophenyl)quinoxaline (FPQx)) and thiophene (T) were used as the donor (D) unit, acceptor (A) unit and π-bridge, respectively. Compared to the non-fluorine substituted PIDT-DTPQx, fluorine substituted PIDT-DTFPQx presents a deep HOMO energy level and a high hole mobility. Obviously, improved the Voc, Jsc, and FF simultaneously, giving rise to overall efficiencies in the PIDT-DTFPQx/PC71BM-based PSCs. A highest PCE of 5.78% was obtained with a Voc of 0.86 V, Jsc of 10.84 mA cm−2 and FF of 61.7% in the PIDT-DTFPQx/PC71BM-based PSCs, while PIDT-DTPQx based devices also demonstrated a PCE of 5.11%, under the illumination of AM 1.5G (100 mW cm−2). Note that these PCE values were achieved for PSCs without any extra treatments. Furthermore, these optimal devices have a film thickness of about 175 nm for the polymer/PC71BM-based active layers. The results provide that introduction of the fluorine atom into quinoxaline unit by side-chain engineering is one of the effective strategies to construct the promising polymer donor materials for future application of large-area polymer solar cells.

N-Type Water/Alcohol-Soluble Naphthalene Diimide-Based Conjugated Polymers for High-Performance Polymer Solar Cells.

With the demonstration of small-area, single-junction polymer solar cells (PSCs) with power conversion efficiencies (PCEs) over the 10% performance milestone, the manufacturing of high-performance large-area PSC modules is becoming the most critical issue for commercial applications. However, materials and processes that are optimized for fabricating small-area devices may not be applicable for the production of high-performance large-area PSC modules. One of the challenges is to develop new conductive interfacial materials that can be easily processed with a wide range of thicknesses without significantly affecting the performance of the PSCs. Toward this goal, we report two novel naphthalene diimide-based, self-doped, n-type water/alcohol-soluble conjugated polymers (WSCPs) that can be processed with a broad thickness range of 5 to 100 nm as efficient electron transporting layers (ETLs) for high-performance PSCs. Space charge limited current and electron spin resonance spectroscopy studies confirm that the presence of amine or ammonium bromide groups on the side chains of the WSCP can n-dope PC71BM at the bulk heterojunction (BHJ)/ETL interface, which improves the electron extraction properties at the cathode. In addition, both amino functional groups can induce self-doping to the WSCPs, although by different doping mechanisms, which leads to highly conductive ETLs with reduced ohmic loss for electron transport and extraction. Ultimately, PSCs based on the self-doped WSCP ETLs exhibit significantly improved device performance, yielding PCEs as high as 9.7% and 10.11% for PTB7-Th/PC71BM and PffBT4T-2OD/PC71BM systems, respectively. More importantly, with PffBT4T-2OD/PC71BM BHJ as an active layer, a prominent PCE of over 8% was achieved even when a thick ETL of 100 nm was used. To the best of our knowledge, this is the highest efficiency demonstrated for PSCs with a thick interlayer and light-harvesting layer, which are important criteria for eventually making organic photovoltaic modules based on roll-to-roll coating processes.

Morphology construction of vertical phase separation for large-area polymer solar cells

Although the power conversion efficiency (PCE) of small-area polymer solar cells (PSCs) has now surpassed 10% upon various optimizations, it is still a challenge to improve the performance of large-area device partially due to the problems encountered in film morphology optimization and incompatibility of using these optimizations for size scale-up. Herein, we reported a method to effectively improve the morphology of photoactive layer and thus device efficiency under ambient atmosphere by using spray coating technique. It was found that by finely adjusting the parameters of the instrument, and the properties of the “electronic ink”, the dynamics of solvent evaporation after film deposition could be controlled. By resorting to a few techniques for morphology characterization, it confirmed that the resulting blend film showed desirable vertical phase separation where polymer enriched near the anode and PCBM adjacent to the cathode. The graded composition is beneficial for the efficient charge transportation as well as eventual charge collection. By using PBDT-TFQ/PC71BM composite as an example, the exceptional high PCEs of 4.6% and 4.1% were achieved for the devices with the size of 1.0 and 10.2cm2, respectively. In comparison to conventional device fabrication process, this method is not only simple without additional treatment steps, but also dramatically increases the device efficiency, which opens a new way for fabricating highly efficient large-area PSCs.

High-Performance Polymer Solar Cells with Electrostatic Layer-by-Layer Self-Assembled Conjugated Polyelectrolytes as the Cathode Interlayer.

An easy and efficient approach to achieve a large-area cathode interlayer with controlled film composition, uniformity, and thickness under a nanometer scale is reported by using an electrostatic layer-by-layer (eLbL) self-assembly process. The eLbL films provide a new means for preparing efficient interlayers for polymer solar cells (PSCs) and also represent a potential candidate for use in high-performance large-area PSC modules in the future.

Performance improvement of large-area roll-to-roll slot-die-coated inverted polymer solar cell by tailoring electron transport layer

For attaining the commercialization, the large-area roll-to-roll (R2R) slot-die-coated inverted polymer solar cells (PSCs) would be promising. The electron transport layer (ETL) plays an important role in scale-up development from small PSCs. In this work, the active layer comprising poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and sol–gel zinc oxide (ZnO) ETL were slot-die coated on flexible PET/ITO substrate as reference based on the R2R process. Aluminum-doped ZnO (AZO) instead of ZnO was also adopted. For further improving the performance of large-area PSC, we used polyethylenimine ethoxylated (PEIE) material to tailor the nanostructure of ZnO and AZO ETLs for reducing the surface traps on ETLs using two approaches: (1) ZnO/PEIE bilayer formed by sequential deposition and (2) ZnO:PEIE hybrid composite as ETLs. The present work would demonstrate the optimum material (fabrication) parameters and how the performance of the R2R large-area slot-die coated PSC can be improved by modifying the ZnO- and AZO-based ETL structures. We also perform the various measurements and structural characterization to understand the relationship among R2R slot-die coating process, charge transport properties, film structure and photovoltaic performance as increasing the device areas from 1×0.3cm2 (power conversion efficiency: 2.69%) to 1×1cm2.

Modeling and simulation of bulk heterojunction polymer solar cells

This review summarizes the optical and electrical models of bulk heterojunction (BHJ) polymer solar cells (PSCs) and numerically simulates and analyzes the performance of the PSCs. A complete simulation of a conventional BHJ device based on the polymer P3HT is presented and results are compared with the experimental data. Key factors affecting the device performance, including the photo absorption, quantum efficiency, short-circuit current, fill factor, and open-circuit voltage of the device, are analyzed and summarized. Simulations on inverted, semitransparent, and large-area PSCs are performed and findings are compared with experimental results. Simulations reveal the effects of optical spacer layers, different thicknesses, carrier mobilities, light intensities, contact barriers, effective bandgaps, recombination coefficients, and energy-level bending on the quantum efficiency, short-circuit current, fill factor, and open-circuit voltage of the PSCs. Differences between conventional and inverted geometry, opacity and semitransparency, and small and large-area PSCs are discussed based on the simulations. A power conversion efficiency of 11.0% is predicted for the PSC based on P3HT. Results suggest the need to further reduce the series resistance in large-area PSCs.

Effect of solvent on large-area polymer–fullerene solar cells fabricated by a slot-die coating method

We developed bulk-heterojunction (BHJ) polymer solar cells (PSCs) by slot-die coating a poly(3-hexylthiophene):fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) composites with various solvents, such as chlorobenzene (CB), dichlorobenzene (DCB), or a CB/DCB co-solvent. To understand the effect of solvent on the uniform active layer film coating and charge carrier recombination, we characterized the coated films with UV–visible absorption spectroscopy, and analyzed the measured light intensity dependence of the cells׳ short-circuit current density (Jsc) and open-circuit voltage (Voc). The PSC with the active layer that was made with a CB/DCB co-solvent shows better uniform film quality and power conversion efficiency (PCE) due to its relatively small bimolecular and trap-assisted recombination. Finally, we successfully fabricated a uniform large-area PSC module from P3HT:PCBM with a CB/DCB co-solvent system, which shows a PCE of 1.73% under the condition of air mass 1.5.

A New Architecture for Printable Photovoltaics Overcoming Conventional Module Limits

A new architecture for manufacturing large-area polymer solar cells that does not produce concomitant aperture and Ohmic losses is presented. By introducing the innovative concept of metal-filamentary nanoelectrodes, which are vertically formed inside the main active layers, loss-free, widely expandable solar cells with the highest relative power conversion efficiency (ca. 90%) in organic photovoltaic systems are demonstrated.

Development of Large Band-Gap Conjugated Copolymers for Efficient Regular Single and Tandem Organic Solar Cells

We demonstrated the synthesis and characterization of two conjugated copolymers, PBDTFBZO and PBDTFBZS, consisting of dialkylthiol substituted benzo[1,2-b:4,5-b']dithiophene donor and monofluorinated benzotriazole acceptor blocks. The resulting copolymers show large band gaps, deep HOMO and LUMO energy levels. Improved V(oc), J(sc), and FF were obtained at the same time to increase overall efficiencies of their single and tandem polymer solar cells. The enhanced V(oc) can be ascribed to a low-lying HOMO energy level by incorporating dialkylthiol and fluorine substituents on the polymer backbone. The improvement in J(sc) and FF are likely due to high carrier mobility, suppressed charge recombination, and fine nanostructure morphology. A 7.74% PCE was achieved from the regular single device based on PBDTFBZS:PC71BM blend film with 3% 1,8-diiodooctane (DIO) additive. In combination with low band gap diketopyrrolopyrrole (DPP)-based copolymer, tandem devices based on PBDTFBZS exhibited high PCE up to 9.40%. The results indicate that PBDTFBZO and PBDTFBZS are promising polymer donor materials for future application of large-area polymer solar cells.

High-performance ITO-free spray-processed polymer solar cells with incorporating ink-jet printed grid

Highly efficient ITO-free polymer solar cells (PSCs) based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been fabricated by a combination of inkjet-printing and spray processes. A hybrid transparent conducting electrode consisting of printed silver (Ag) grids and highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PH1000) was used as an alternative to indium-tin oxide (ITO). Spray process incorporating with printed Ag grids played a critical role in improving the interfacial contact between Ag grids and photoactive layer, and thus enhanced the performance of ITO-free large-area PSC. The ITO-free PSC (device area=0.3cm2) prepared here has a comparable performance of 2.86%. The average PCE of 2.34% was achieved in the ITO-free PSC with a large electrode area (8cm2) fabricated herein by the combination of inkjet-printed grid and spray processes. This result is much better than ITO-based large-area PSC generally reported.

Retention of Power Conversion Efficiency – From Small Area to Large Area Polymer Solar Cells (Adv. Mater. 15/2013)

Retention of Power Conversion Efficiency – From Small Area to Large Area Polymer Solar Cells (Adv. Mater. 15/2013)

Facile hot solvent vapor annealing for high performance polymer solar cell using spray process

In this paper, the use of hot solvent vapor (HSV) annealing is demonstrated to improve the performance of polymer solar cells (PSCs) fabricated by the spray process. The blend of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM) is used as a photoactive layer of the solar cell. Compared to the conventional solvent vapor annealing, the HSV annealing can eliminate the formation of intra-droplet boundary during the spray process and optimize the nanostructure of the film quickly (≤5min). The study of conducting atomic force microscopy (C-AFM) reveals that the HSV annealed film exhibits smooth film surface and homogenous conductivity distribution. Moreover, an enhanced light harvesting and increased crystallinity of P3HT in the active layer are observed by UV–vis absorption and X-ray diffraction (XRD). With subsequent thermal annealing, the power conversion efficiency of solar cell made from the HSV annealed film is reached at 3.61%. This HSV annealing technique can be implemented into the fabrication of high efficient large-area PSCs using the spray process.

Slot-die coating parameters of the low-viscosity bulk-heterojunction materials used for polymer solarcells

Slot-die coating is one of the promising methods for fabricating large-area polymer solar cells (PSCs) due to its compatibility with roll-to-roll printing and one-dimensional direct patterning. In this study, we investigated correlations between conformal film morphology of intermixed poly(3-hexylthiophene) (P3HT) and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk-heterojunction (BHJ) material and universal control factors such as shim length, substrate temperature, and coating speed. The observed correlation equation is better matched to the meniscus coating parameter equation rather than to the traditional high-viscosity extrusion slot-die coating parameter equation. Based on our findings regarding the governing factors of the thin film formation of bulk-heterojunction materials, we demonstrated the development of uniform large-area polymer solar cells with power conversion efficiency (PCE) of 3.07% under Air Mass 1.5condition and the possibility of high-throughput production.

Retention of Power Conversion Efficiency – From Small Area to Large Area Polymer Solar Cells

A light harvesting device geometry is implemented for polymer solar cells that retain efficiency when the device area is scaled up. Patterning devices and incorporating suitable fluorescent dye doped polymers in the spaces between the patterns leads to 12% efficiency enhancement. Vacuum free deposition of electrodes is carried out using meltable alloys giving rise to device efficiency of 6%. The fluorescent layer along with the electrode serves as an active encapsulant leading to improved device stability.