Amphiphilic fullerene derivative as effective interfacial layer for inverted polymer solar cells

Amphiphilic fullerene derivative with poly(ethylene glycol) chain (C60-PEG) was applied as effective interfacial layer to improve the performance of inverted polymer solar cells. C60-PEG could not only be used as cathode buffer layer alone by replacing ZnO, but also be used as a self-assembled monolayer to modify ZnO. C60-PEG can tune energy level alignment and improve the interfacial compatibility between active layer and ITO or ZnO. Moreover, due to the strong interaction between ZnO nanoparticles and PEG chain, C60-PEG can passivate the surface defects and traps of ZnO, and facilitate the charge selective and dissociation. Consequently, inverted polymer solar cells based on thieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]- phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) present a PCE of 6.6% by incorporating C60-PEG into as cathode buffer layer. Moreover, an improved PCE of 7.4% with good long-term stability in air were further achieved by using C60-PEG/ZnO interlayer. In this work, C60-PEG could be prepared by solution process at room temperature without additional annealing, which shows the potential in large-scale printed polymer solar cells.     

Efficiency improvement of polymer solar cells by iodine doping

A series of poly(3-hexylthiophene) (P3HT)/(6,6)-phenyl C₆₀ butyric acid methyl ester (PCBM) bulk hetero-junction polymer solar cells were fabricated with different iodine (I₂) doping concentrations. The short circuit current density (J_sc) was increased to 8.7 mA/cm² from 4 mA/cm², meanwhile the open circuit voltage (V_oc) was decreased to 0.52 V from 0.63 V when the iodine doping concentration is 5%. The optimized power conversion efficiency of polymer solar cells (PSCs) with iodine doping is about 1.51%, which should be attributed to the better charge carrier transport and collection, and the more photon harvesting due to the red shift of absorption peaks and the widened absorption range to the longer wavelength. The morphology and phase separation of polymer thin films were measured by atomic force microscopy (AFM). The phase separation of P3HT and PCBM has been distinctly increased, which is beneficial to the exciton dissociation. The photocurrent density of PSCs with iodine doping was increased compared with the PSCs without iodine doping under the same effective voltage.     

Solution-processed vanadium oxide as an anode interlayer for inverted polymer solar cells hybridized with ZnO nanorods

Solution-processed vanadium oxide (V₂O₅) as an anode interlayer is introduced between the organic layer and the Ag electrode for improving the performance of the low-cost inverted polymer solar cells hybridized with ZnO nanorods. Our investigations indicate that the solution-processed V₂O₅ interlayer as an electron-blocking layer can effectively prevent the leakage current at the organic/Ag interface. The power conversion efficiency is improved from 2.5% to 3.56% by the introduction of the V₂O₅ interlayer. The V₂O₅ interlayer also serves as an optical spacer to enhance light absorption, and thereby increases the photocurrent. Compared to the vacuum-deposited techniques, the fabrication of the solution-processed V₂O₅ interlayer is simple and effective. The solution-based approach makes it attractive for applications to mass production and potentially printed organic electronics.     

Utilizing intermixing of conjugated polymer and fullerene from sequential solution processing for efficient polymer solar cells

Efficient polymer solar cells based on poly[2, 6-(4, 4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-fluorobenzothiadiazole)] (PCPDTFBT) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) are successfully fabricated by a sequential processing (SqP). With appropriate orthogonal solvent and thermal treatment, the SqP film can form an inter-diffused layer, and the SqP devices show efficient photovoltaic performance in both conventional and inverted layouts. The SqP inverted device was firstly demonstrated and the highest power conversion efficiency (PCE) of 5.84% with the enhanced J_sc of 16.4 mA cm⁻² was able to be achieved with the high internal quantum efficiency (IQE). Photoluminescence quenching shows the SqP films can provide efficient exciton quenching. X-ray photoemission spectroscopy (XPS) and ellipsometry analysis shows a polymer-rich surface in SqP films after thermal annealing. The charge mobilities in the SqP films were significantly enhanced as measured by space-charge-limited-current (SCLC) method. All these contribute to the improved photovoltaic performance in the inverted SqP device. We believe that these results inspire a new way of forming the active layer with controllable morphology, efficient charge separation and collection in polymer solar cells.     

High-performance inverted polymer solar cells with lead monoxide-modified indium tin oxides as the cathode

The photovoltaic stability of polymer solar cells (PSCs) can be greatly improved by adopting an inverted device structure. This paper reports high-performance inverted PSCs with lead monoxide (PbO)-modified indium tin oxide (ITO) as the cathodes. A thin PbO layer can effectively lower the work function of ITO from 4.5 to 3.8 eV. The optimal inverted PSCs with poly(3-hexylthiophene) (P3HT) as the donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor exhibited high photovoltaic performance: open-circuit voltage of 0.59 V, short-circuit current density of 10.8 mA cm⁻², fill factor of 0.632, and power conversion efficiency of 4.00% under simulated AM1.5G illumination (100 mW cm⁻²). The photovoltaic efficiency is significantly higher than that of the control inverted PSCs with unmodified ITO as the cathode. It is even better than that of the control PSCs with normal architecture, which have an optimal efficiency of 3.5%. The lowering in the work function by the PbO modification is attributed to the charge transfer between PbO and ITO, as evidenced by the X-ray photoelectron spectra.     

The effect of processing additive on aggregated fullerene derivatives in bulk-heterojunction polymer solar cells

The effects of processing additive on fullerene aggregation in polymer BHJ solar cells were investigated using new fullerene derivatives bearing a thiophene moiety and alkyl groups. Although new fullerene derivatives showed quite similar electronic transport properties in field-effect transistors, the photovoltaic performances were significantly limited by their aggregative nature. Processing with 1% CN additive, however, changed the aggregated morphology of BHJ films to a smoother and homogeneous morphology, improving photovoltaic performance. The result indicates that processing additive not only influences on polymer side, but also significantly affects fullerene acceptor component.     

Efficient and stable inverted polymer solar cells using TiO2 nanoparticles and analysized by Mott-Schottky capacitance

The performance of both inverted and conventional polymer solar cells (PSCs) were examined with a low-temperature, solution-processed synthesized TiO₂ nanoparticles (TiO₂ NPs) as the electron extraction layer. The performance of inverted PSCs based on P3HT:PCBM bulk-heterojunction with a TiO₂ NPs layer was dramatically improved and the highest power conversion efficiency (PCE) of 4.56% was achieved via 24 h exposure in air, which is one of the highest PCEs for P3HT:PCBM bulk-heterojunction PSCs using TiO₂ as electron extraction layer. Meanwhile, the performance of inverted PSCs was superior to regular PSCs. Mott-Schottky capacitance analysis was carried out for both inverted and regular PSCs to obtain the built-in potential, the depletion width, as well as the doping level of the active layer, which all support the performance improvement of PSCs devices with inverted structure. In addition, inverted PSCs show excellent stability in air without encapsulation. The PCE can retain 87% of its original values after 400 h exposure in air, which is much better than that of regular PSCs. The results indicate that solution-processed TiO₂ NPs shows great potential applications in the fabrication of highly efficient and stable inverted PSCs as well as large-area, flexible printed PSCs.     

Light-trapping in polymer solar cells by processing with nanostructured diatomaceous earth

We demonstrate the use of fossilized diatoms (diatomaceous earth) as light traps in regioregular poly(3-hexylthiophene) (P3HT) and fullerene derivative [6,6]-phenyl-C₆₀-butyric acid methyl ester (PCBM) solar cells. Diatoms, the most common type of phytoplankton found in nature, are optimized for light absorption through millions of years of adaptive evolution. They are also an earth-abundant source of silica that can be incorporated into polymer solar cells without the need for complicated processing. Here we establish protocols dispersing the diatomaceous earth throughout the P3HT:PCBM active layer with characterization by optical and current-voltage measurements. We show that through the addition of diatomaceous earth, we can achieve the same power conversion efficiencies as standard thickness cells while using 36% thinner active layers. We find that adding the diatomaceous earth acts as a scattering center and textures the silver back contact, contributing to increases in the optical path length within devices. Results from this study open up pathways for incorporating hierarchical materials from nature into energy conversion devices.     

Efficient and ultraviolet durable inverted polymer solar cells using thermal stable GZO-AgTi-GZO multilayers as a transparent electrode

The optical and electrical properties of GZO/AgTi/AZO (GATG) multilayer transparent conducting films fabricated by magnetron sputtering method were investigated. The sheet resistance and maximum optical transmittance of GATG films are 5 Ω/sq and 86%, respectively. The sheet resistance of GATG still retains stable under annealing at 400 °C, which shows better thermal stability compared to GZO/Ag/AZO (GAG) film. The enhanced thermal stability of GATG is attributed to the formation of TiO_X in Ti doped Ag nanostructure film, which can inhibit Ag atom diffusion and aggregation. PTB7-TH:PC₇₁BM based inverted polymer solar cells (PSCs) with GATG electrode gave PCE of 9.20%, which is comparable to PCE (9.23%) of the control PSCs with ITO electrode. The PCE of PSCs with GATG and ITO electrodes respectively remain 59% and 23% of the original PCE values after UV exposure for 20 min with relativize humidity of 68% in air, indicating that PSCs with GATG show better UV durability. Our results suggest that GATG as an alternative to ITO electrode can obtain efficient inverted PSCs and have stronger anti-UV ability due to its low UV transparency.     

Rubrene-based interfacial engineering toward enhanced performance in inverted polymer solar cells

Rubrene, an organic semiconductor having stable fused-ring molecular structure was used as a double interfacial layer in inverted organic solar cells. When a thin, 3 nm-thick layer of rubrene was introduced between a MoO₃-based hole-collecting layer and a bulk-heterojunction (BHJ) photo-active layer consisting of poly{4,8-bis[(2-ethylhexyl)oxy]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) and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM), the power conversion efficiency was improved over 12% (from 7.2% to 8.1%). It was demonstrated that the insertion of thin rubrene layer showed suppressed exciton quenching and improved exciton dissociation, resulting in more efficient charge carrier collection and weaker charge recombination, thus improving the device performance.     

Highly conductive Zinc-Tin-Oxide buffer layer for inverted polymer solar cells

Thin-films of Zinc Tin Oxide (ZTO) with an extremely high charge carrier mobility and superior optical transmittance are synthesized using a simple solution method. These ZTO films have been systematically studied for the application in inverted polymer solar cells (PSCs). The Hall effects measurements show that the charge mobility of the ZTO semiconductor is over 16.5 cm².V⁻¹.S⁻¹, which is the highest mobility value ever reported for oxide buffer made by using solution process. By applying the ZTO buffer layer in the inverted PSCs of P3HT:PC₆₁BM, the power conversion efficiency of the device is 30% higher than that of the devices made with other common buffer layers such as ZnO and TiO₂. Light intensity-dependent JV studies and PL measurements also indicate that ZTO buffer layer reduces surface recombination. This work demonstrates that the solution-synthesized ZTO is a promising new buffer layer with superior electron extraction capability for the solar cells.     

Performance improvement of pentacene-doped P3HT:PCBM inverted polymer solar cells with AZO nanorod array passivated using photoelectrochemical technique

The pentacene-doped P3HT:PCBM inverted polymer solar cells (IPSCs) with Al-doped zinc oxide (AZO) nanorod array were fabricated. The AZO nanorod array could enhance the carrier collection and carrier extraction capability. The AZO nanorod array formed by the laser interference photolithography method and the wet etching process sequentially was used as the carrier collection and carrier transportation layers. The defects on the sidewall surface of the AZO nanorods were passivated by using the photoelectrochemical (PEC) method. It was demonstrated that the better performance of the IPSCs was obtained by PEC treatment. Compared with the IPSCs without PEC treatment, the short current density and power conversion efficiency of the IPSCs with PEC treatment for 60 s increased from 14.56 mA/cm² to 15.85 mA/cm² and 5.45% to 6.13%, respectively.     

Efficiency improvement of polymer solar cells with random micro-nanostructured back electrode formed by active layer self-aggregation

The formation of a micro-nanostructured back electrode provides an efficient route for enhancing light absorption in polymer solar cells by light scattering of the bumpy electrode. In this study, we incorporated propylene glycol mono-methyl ether acetate (PGMEA) into the poly (3-hexylthiophene) (P3HT) and [6:6]-phenyl-C61-butyric acid (PC₆₁BM) solution, and the PGMEA induced P3HT aggregations give rise to a bumpy surface of the active layer. The sequential deposition of the Al electrode onto the active layer creates a polymer solar cell with interpenetrated micro-nanostructured morphology of the active layer/electrode interface. The higher crystallinity of P3HT induced by active layer self-aggregation improves carrier mobility. The bumpy active layer/electrode interface can not only facilitate charge carriers transfer and collection in the device, but also enhance optical scattering and leads to enhanced light absorption of the active layer. The resulting device shows improved photocurrent, corresponding to power conversion efficiency improvement of 17.9% as compared to the planar device. This work indicates that the active layer self-aggregation is a simple, cost-effective and mold-free methodology to manufacture high performance polymer solar cells with the micro-nanostructured back electrode.     

Significant improvement of photocurrent in dye-sensitized solar cells by incorporation thiophene into electrolyte as an inexpensive and efficient additive

For the first time, thiophene as an efficient additive is applied in electrolyte of dye-sensitized solar cells (DSSCs). In the cell based on 2-cyano-3-(4-(diphenylamino)phenyl)acrylic acid (TPA) sensitizer, addition of 1 M thiophene to the electrolyte (instead of 4-tert-butylpyridine (TBP)) increases significantly photocurrent density (J_sc) from 4.62 to 7.80 mA cm⁻². Consequently, overall conversion efficiency (η) enhances from 2.17% to 3.18% with a 46% improvement. For DSSC based on 2-cyano-3-(2′-(5′,10′,15′,20′-tetraphenylporphyrinato zinc(II))yl)acrylic acid (Zn-1) with modified electrolyte, J_sc increases from 4.18 to 8.23 mA cm⁻² and η improves from 1.46% to 1.96% that shows an enhancement of 34% in η. Intensity modulated photocurrent spectroscopy (IMPS) indicates the faster electron transfer in the DSSC based on thiophene additive compared to the standard device. Also, the higher electron diffusion coefficient (D_n) of the DSSC with modified electrolyte shows the thiophene additive facilitates the electron transfer. Based on cyclic voltammetry (CV) and UV–vis data, thiophene forms a molecular complex with iodine molecule in the electrolyte solution that has a remarkable effect on the redox couple oxidation–reduction reactions which improves J_sc. A significant improvement in J_sc with a small decrease in V_oc and thereby an increase in η are observed from addition thiophene.     

Polyacetylene-based polyelectrolyte as a universal interfacial layer for efficient inverted polymer solar cells

Here we report that poly(N-dodecyl-2-ethynylpyridiniumbromide) (PDEPB) interlayers between electron-collecting zinc oxide (ZnO) layers and bulk heterojunction (BHJ) layers act as a universal interfacial layer for improving the performances of inverted-type polymer:fullerene solar cells. Three different BHJ layers, poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), poly[(4,8-bis(2-ethylhexyloxy)-benzo[1,2-b:4,5-b']dithiophene)-2,6-diyl-alt-(N-2-ethylhexylthieno[3,4-c]pyrrole-4,6-dione)-2,6-diyl]] (PBDTTPD):PC₆₁BM, and poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), were employed so as to prove the role of the PDEPB interlayers. Results showed that the power conversion efficiency (PCE) of polymer:fullerene solar cells with the three different BHJ layers increased in the presence of the PDEPB interlayers prepared from 0.5 mg/ml solutions. The improved PCE was attributed to the conformal coating of the PDEPB layers on the ZnO layers (by atomic force microscopy measurement), lowered work functions of ZnO induced by the PDEPB layers (by Kelvin probe measurement), and reduced interface resistance (by impedance spectroscopy measurement), as supported by the noticeable change in the atom environments of both the ZnO and PDEPB layers (by X-ray photoelectron spectroscopy measurement).     

Zwitterionic ammonium and neutral amino molecules as cathode interlayer for inverted polymer solar cells

Cathode interlayer is essential to inverted bulk heterojunction polymer solar cells (PSCs). A series of zwitterionic ammonium and neutral amino organic molecules are introduced into inverted PSCs as cathode interlayer and power conversion efficiency (PCE) as high as 8.07% is demonstrated. Compared to the devices without interlayer, all the devices exhibit significant improvements of the device parameters by reducing the work function of indium tin oxide (ITO) cathode. It is striking that the devices with neutral amino molecules as interlayer exhibit remarkably higher PCEs than the devices with zwitterionic ammonium molecules as interlayer. We attribute the improved performance to the better photoactive morphology induced by the hydrophobic properties of the neutral amino derivatives through research of ultraviolet photoelectron spectroscopy, atomic force microscopy, and contact angle measurements. Interestingly, the PCEs of the inverted PSCs with cathode interlayer are positively correlated with the hydrophobic properties of the interlayer materials, since devices with neutral amino molecules or molecules with a more hydrophobic alkyl pendant (piperidine) as interlayer exhibit higher PCEs. These results pave the way to the design of effective cathode interlayer materials.     

In-situ synthesis of metal nanoparticle-polymer composites and their application as efficient interfacial materials for both polymer and planar heterojunction perovskite solar cells

A new approach for the synthesis of gold nanoparticles (Au NPs) via a simple and fast in-situ generation method using an amine-containing polymer (PN4N) as both stabilizer and reducing agent is reported. The application of the Au NPs-PN4N hybrid material as efficient interfacial layer in different types of solar cells was also explored. The synthesized Au NPs show good uniformity in size and shape and the Au NPs doped PN4N hybrid composites exhibit high stability. Amine-containing polymers are good cathode interfacial materials (CIMs) in polymer solar cells (PSCs) and planar heterojunction perovskite solar cells (PVKSCs). The performance of the PSCs with Au NPs doped PN4N CIMs is largely improved when compares to devices with pristine PN4N CIM due to the enhanced electronic properties of the doped PN4N. Furthermore, by incorporating larger Au NPs into PEDOT:PSS to enhance absorption of the light harvesting layer, power conversion efficiencies (PCEs) of 6.82% and 13.7% are achieved for PSC with PCDTBT/PC₇₁BM as the light harvesting materials and PVKSC with a ∼280 nm-thick CH₃NH₃PbI_3−xCl_x perovskite layer, respectively. These results indicate that Au NPs doped into both PEDOT:PSS and PN4N interlayers exhibited a synergistic effect in performance improvement of PSCs and PVKSCs.     

Enhanced efficiency and stability of polymer solar cells with TiO2 nanoparticles buffer layer

TiO₂ sols synthesized with a facile solution-based method were used as a buffer layer between the active layer and the cathode Al in conventional structure polymer solar cells (PSCs). Using transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) and atomic force microscopy (AFM), the morphological and crystallographic properties of synthesized TiO₂ nanoparticles (TiO₂ NPs) as well as the buffer layer were studied in detail. It was observed that by increasing H₂O in the process of peptization both the crystallinity and particle size of TiO₂ NPs were enhanced, while the particles in sol showed a narrower size distribution conformed by dynamic light scattering. Inserting TiO₂ NPs as a buffer layer in conventional structure PSCs, both the power conversion efficiency (PCE) and stability were improved dramatically. PSCs based on the structure of ITO/PEDOT:PSS/P3HT:PCBM/TiO₂ NPs/Al showed the short-circuit current (J_sc) of 12.83 mA/cm² and the PCE of 4.24%, which were improved by 31% and 37%, respectively comparing with the reference devices without a TiO₂ buffer layer. The stability measurement showed that PSC devices with a TiO₂ NPs buffer layer could retain 80% of the original PCEs after exposed in air for 200 h, much better than the devices without such a buffer layer. The effect can be attributed to the protection by the buffer layer against oxygen and H₂O diffusion into the active layers. The observations indicate that TiO₂ NPs synthesized by facile solution-based method have great potential applications in PSCs, especially for large-area printed PSCs.     

Ionic liquids with variable cations as cathode interlayer for conventional polymer solar cells

Interfacial layer materials have been demonstrated to be crucial for high-efficiency polymer solar cells (PSCs). In this work, we use ionic liquid (IL) as cathode interfacial layer (CIL) for highly efficient conventional PSCs (c-PSCs) and investigate functions of ILs with different cations and substituents. Employing IL as the CIL, PBDTTT-C:PC₇₁BM-based c-PSC affords a power conversion efficiency (PCE) of 7.29%, much higher than that without the CIL (2.78%) and that with Ca/Al electrode (6.18%). When the photoactive layer is a PTB7-Th:PC₇₁BM blend, a higher PCE of 8.67% can be obtained. The ILs reduce the energy barrier due to the existence of interfacial dipole in c-PSCs, leading to increased electron and hole mobilities, reduced series resistance and enhanced contact at the cathode interface. Meanwhile, alkyl chain-substituted ILs offer higher fill factor and PCE than aromatic groups-substituted analogue, which is mainly contributed to more balanced electron and hole mobilities. This work suggests that the ILs are qualified candidates as the CIL for c-PSCs and that one should take the substitution effect into account when choosing a CIL from a large library of materials.     

Transient photovoltage and dark current analysis on enhanced open-circuit voltage of polymer solar cells with hole blocking TiO2 nanoparticle interfacial layer

The open-circuit voltage of bulk heterojunction polymer solar cells utilizing 1,8-diiodooctane (DIO) as a processing additive was greatly improved by using an organic layer coated TiO₂ nanoparticle interfacial layer inserted between the active layer and the Al electrode. The transient photovoltage measurement revealed that there was significant non-geminate recombination at the DIO-processed active layer/Al electrode interface. Reduced open-circuit voltage (V_OC) of the photovoltaic devices and high water contact angle of the DIO-processed active layer showed that the DIO-processed active layer has an undesirable surface composition for the electron collection. The organic layer coated TiO₂ nanoparticle interfacial layer effectively prevented the non-geminate recombination at the active layer/Al interface. As a result, we were able to significantly improve the V_OC and power conversion efficiency from 0.46 V and 2.13% to 0.62 V and 3.95%, respectively.