Polymer solar cells with NiO hole-collecting interlayers processed by atomic layer deposition

We report on the photovoltaic properties of polymer solar cells that use NiO-coated indium tin oxide (ITO) as the hole-collecting electrode. The NiO films were prepared by atomic layer deposition (ALD) on top of ITO with thicknesses varying from 6 to 25 nm. The NiO films increase the work function (WF) of the ITO, allowing NiO-coated ITO to act as an efficient hole-collecting electrode. Devices made with pristine NiO showed poor current–voltage characteristics. However, subsequent O₂-plasma treatment further increased the WF of NiO, tuning NiO-coated ITO into an efficient hole-collecting electrode for polymer solar cells based on the donor poly(3-hexylthiophene-2,5-diyl) (P3HT). The polymer solar cells with the O₂-plasma treated NiO-coated ITO hole-collecting electrodes yield a power conversion efficiency of 4.1 ± 0.2% under simulated air mass 1.5 G 100 mW/cm² illumination, which is comparable to reference devices with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)-coated ITO hole-collecting electrodes.     

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).     

Optical,electrical and mechanical properties of indium tin oxide on polyethylene terephthalate substrates: Application in bulk-heterojunction polymer solar cells

We investigated optical, electrical and mechanical properties of indium tin oxide (ITO) on flexible polyethylene terephthalate (PET) substrate, considering bulk-heterojunction (BHJ) polymer solar cells applications. Encapsulation of flexible solar cells with the architecture PET/ITO/PEDOT:PSS/P3HT:PCBM (or P3HT:PCBM:AZ-NDI-4)/Al was done by direct brush-painting with nail enamel. Active cell layer blends of [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) with regioregular or regiorandom poly(3-hexylthiophene-2,5-diyl) (P3HT) were applied. Additionally for this role the mixture of regioregular P3HT:PCBM with naphthalene diimide–imine with four thiophene rings AZ-NDI-4 was tested. Obtained photovoltaic (PV) and optical (UV–vis) results of the flexible polymer solar cells were compared with the same architecture of devices on the glass/ITO substrate.     

A plasmonically enhanced polymer solar cell with gold–silica core–shell nanorods

We report the use of chemically synthesized gold (Au)–silica core–shell nanorods with the length of 92.5 ± 8.0 nm and diameter of 34.3 ± 4.0 nm for the efficiency enhancement of bulk heterojunction (BHJ) polymer solar cells. Silica coated Au nanorods were randomly blended into the BHJ layers of these solar cells. This architecture inhibits the carrier recombination at the metal/polymer interface and effectively exploits light absorption at the surface plasmon resonance wavelengths of the Au–silica nanorods. To match the two plasmon resonant peaks of the Au–silica nanorods, we employed a low bandgap polymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) to construct a solar cell. The absorption spectrum of PCPDTBT:[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM) is relatively wide and matches the two plasmon resonance peaks of Au–silica nanorods, which leads to greater plasmonic effects. We also constructed the poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PC₆₀BM) cells for comparison. The absorption spectrum of P3HT:PC₆₀BM only overlaps one of the plasmon resonance peak of Au–silica nanorods. The efficiency of the P3HT:PC₆₀BM device incorporating optimized Au–silica nanorods is enhanced by 12.9% from 3.17% to 3.58%, which is due to the enhanced light absorption. Compared with the P3HT:PC₆₀BM device with Au–silica nanorods, the PCPDTBT:PC₇₀BM device with 1 wt% Au–silica nanorods concentration has a higher efficiency of 4.4% with an increase of 26%.     

金属表面等离激元增强聚合物太阳电池

研究了Au纳米颗粒表面等离激元增强聚噻吩(P3HT)与富勒烯衍生物(PCBM)共混体系聚合物太阳电池的光电转换效率。Au纳米颗粒表面由双十烷基二甲基溴化铵(DDAB)修饰,能够均匀分散在活性层中。研究了Au纳米颗粒的质量分数对电池性能的影响,发现质量分数为1.2%时,电池性能最佳,转换效率高达3.76%,较未掺杂的参比电池相对提高约20%。掺入Au纳米颗粒后P3HT和PCBM共混膜光吸收显著增强,从而使电池外量子效率大大增加。电池效率的提升主要归结于Au纳米颗粒表面等离激元激发所引起的近场增强。     

Highly Sensitive Narrowband Photomultiplication-Type Organic Photodetectors Prepared by Transfer-Printed Technology

Narrowband photomultiplication-type organic photodetectors (PMOPDs) are realized with poly(3-hexylthiophene-2,5-diyl) (P3HT) as the optical field adjusting (OFA) layer and transfer-printed P3HT: [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) (50:1, w/w) as the photomultiplication (PM) layer. The thickness of the OFA layers is adjusted to optimize interfacial trapped electron distribution and density, which determines the external quantum efficiency (EQE) and spectral response range of PMOPDs. Narrowband PMOPDs with 2.5 µm thick P3HT as the OFA layer exhibit two narrow response peaks at 350 and 660 nm, and the corresponding EQE values at 350 and 660 nm are 180% and 760% under an applied bias of −20 V. A wide bandgap polymer poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (P-TPD) is deliberately incorporated into OFA layer for adjusting interfacial trapped electron distribution near Al electrode. Narrowband PMOPDs exhibit only one response peak at 660 nm with the enhanced EQE value of 1120% under the same bias. The enhanced EQE of PMOPDs with P-TPD is primarily attributed to the increased hole tunneling injection and transport, which can be ascribed to the enhanced trapped electron density near the Al electrode and the improved hole mobility, respectively. Clearly resolved images can be obtained from the imaging system with the narrowband PMOPDs as sensing pixel without any current preamplifier, indicating the promising potential of PMOPDs in imaging sense.     

Efficient ternary blend polymer solar cells with a bipolar diketopyrrolopyrrole small molecule as cascade material

We present a ternary strategy to enhance the power conversion efficiency (PCE) of bulk heterojunction polymer solar cells (PSCs) with a bipolar small molecule as cascade material. A bipolar diketopyrrolopyrrole small molecule (F(DPP)₂B₂), as the second electron acceptor, was incorporated into poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C₆₁-butyric-acidmethyl-ester (PC₆₁BM) blend to fabricate ternary blend PSCs. The introduction of the bipolar compound F(DPP)₂B₂ can not only broaden the light absorption of the active layer because of its absorption in near infrared region but also play a bridging role between P3HT and PC₆₁BM due to the cascaded energy level structure, thus improving the charge separation and transportation. The optimized ternary PSC with 5 wt% F(DPP)₂B₂ content delivered a high PCE of 3.92% with a short-circuit current density (J_sc) of 9.63 mA cm⁻², an open-circuit voltage (V_oc) of 0.62 V and a fill factor (FF) of 64.90%, showing an 23% improvement of PCE as compared to the binary systems based on P3HT:PC₆₁BM (3.18%) or P3HT:F(DPP)₂B₂ (3.17%). The results indicate that the ternary PSCs with a bipolar compound have the potential to surpass high-performance binary PSCs after carefully device optimization.     

Efficient Ternary Organic Solar Cells Enabled by the Integration of Nonfullerene and Fullerene Acceptors with a Broad Composition Tolerance

The ternary structure that combines fullerene and nonfullerene acceptors in a photoactive layer is demonstrated as an effective approach for boosting the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Here, highly efficient ternary OSCs comprising a wide‐bandgap polymer donor (PBT1‐C), a narrow‐bandgap nonfullerene acceptor (IT‐2F), and a typical fullerene derivative (PC₇₁BM) are reported. It is found that the addition of PC₇₁BM into the PBT1‐C:IT‐2F blend not only increases the device efficiency up to 12.2%, but also improves the ambient stability of the OSCs. Detailed investigations indicate that the improvement in photovoltaic performance benefits from synergistic effects of increased photon‐harvesting, enhanced charge separation and transport, suppressed trap‐assisted recombination, and optimized film morphology. Moreover, it is noticed that such a ternary system exhibits excellent tolerance to the PC₇₁BM component, for which PCEs over 11.2% can be maintained throughout the whole blend ratios, higher than that (11.0%) of PBT1‐C:IT‐2F binary reference device.     

Photoconductivity of a Low‐Bandgap Conjugated Polymer

The photoconductive properties of a novel low‐bandgap conjugated polymer, poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)], PCPDTBT, with an optical energy gap of E_g ～ 1.5 eV, have been studied. The results of photoluminescence and photoconductivity measurements indicate efficient electron transfer from PCPDTBT to PCBM ([6,6]‐phenyl‐C61 butyric acid methyl ester, a fullerene derivative), where PCPDTBT acts as the electron donor and PCBM as the electron acceptor. Electron‐transfer facilitates charge separation and results in prolonged carrier lifetime, as observed by fast (t > 100 ps) transient photoconductivity measurements. The photoresponsivities of PCPDTBT and PCPDTBT:PCBM are comparable to those of poly(3‐hexylthiophene), P3HT, and P3HT:PCBM, respectively. Moreover, the spectral sensitivity of PCPDTBT:PCBM extends significantly deeper into the infrared, to 900 nm, than that of P3HT. The potential of PCPDTBT as a material for high‐efficiency polymer solar cells is discussed.     

Near IR Sensitization of Organic Bulk Heterojunction Solar Cells: Towards Optimization of the Spectral Response of Organic Solar Cells

The spectroscopic response of a poly(3‐hexylthiophene)/[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (P3HT/PCBM)‐based bulk heterojunction solar cell is extended into the near infrared region (NIR) of the spectrum by adding the low bandgap polymer poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b´]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)] [PCPDTBT] to the blend. The dominant mechanism behind the enhanced photosensitivity of the ternary blend is found to be a two‐step process: first, an ultrafast and efficient photoinduced charge transfer generates positive charges on P3HT and PCPDTBT and a negative charge on PCBM. In a second step, the positive charge on PCPDTBT is transferred to P3HT. Thus, P3HT serves two purposes. On the one hand it is involved in the generation of charge carriers by the photoinduced electron transfer to PCBM, and, on the other hand, it forms the charge transport matrix for the positive carriers transferred from PCPDTBT. Other mechanisms, such as energy transfer or photoinduced charge transfer directly between the two polymers, are found to be absent or negligible.     

Non-peripheral octahexylphthalocyanine doping effects in bulk heterojunction polymer solar cells

The improvement of long-wavelength sensitivity in bulk heterojunction organic thin-film solar cells based on poly(3-hexylthiophene) (P3HT) by the addition of the soluble phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH₂) is reported. C6PcH₂ possesses near-infrared absorption and can be mixed with a P3HT:1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM) bulk heterojunction active layer. By doping C6PcH₂, the photosensitivity in the long-wavelength region was improved, and the energy conversion efficiency reached 3.0% at a composition ratio of P3HT:C6PcH₂:PCBM = 10:3:10. We discuss the principle of photoconversion in the bulk heterojunction solar cell based on the P3HT:C6PcH₂:PCBM active layer by taking into consideration the existence of both highly ordered P3HT domains and hexagonal columnar structures of C6PcH₂, and the microphase separation of P3HT and C6PcH₂ in the active layer.     

Electrostatic Self‐Assembly Conjugated Polyelectrolyte‐Surfactant Complex as an Interlayer for High Performance Polymer Solar Cells

A simple method is demonstrated to improve the film‐forming properties and air stability of a conjugated polyelectrolyte (CPE) without complicated synthesis of new chemical structures. An anionic surfactant, sodium dodecybenzenesulfonate (SDS), is mixed with cationic CPEs. The electrostatic attraction between these two oppositely‐charged materials provides the driving force to form a stable CPE‐surfactant complex. Compared with a pure CPE, this electrostatic complex is not only compatible with highly hydrophobic bulk‐heterojunction (BHJ) films, e.g. poly(3‐hexylthiophene):[6,6]‐phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM), but also works well with other low bandgap polymer‐based BHJ films. Using this complex as a cathode interface layer, a high power conversion efficiency of 4% can be obtained in P3HT:PCBM solar cells together with improved stability in air. Moreover, ～20% performance enhancement can also be achieved when the complex is used as an interlayer to replace calcium metal for low bandgap polymer‐based BHJ systems.     

Effects of incorporation of copper sulfide nanocrystals on the performance of P3HT: PCBM based inverted solar cells

It has been reported that performance of bulk heterojunction organic solar cells can be improved by incorporation of an additive like metal and semiconducting nanoparticles in the active layer. Here in, we have synthesized Cu₂S nanocrystals (NCs) by chemical route and studied its dispersion in poly (3-hexylthiophene) [6, 6]-phenyl C61-butyric acid methyl ester (P3HT: PCBM) matrix. Variation in the performance parameters with change in the concentration of Cu₂S NCs into the P3HT: PCBM matrix has also been studied and it was found that the inverted geometry device with concentration of 20 wt% of Cu₂S NCs and having the structure ITO/ZnO (NPs)/P3HT: PCBM:Cu₂S NCs/MoO₃/Al has shown maximum efficiency of 3.39% which is more than 100% increase in comparison with devices without Cu₂S NCs. Photoluminescence measurements studies unveiled that the incorporation of Cu₂S NCs into a P3HT: PCBM matrix has helped in quenching photoluminescence which suggests more effective exciton dissociation at the interfaces between the P3HT and PCBM domains. The Nyquist plots obtained from impedance spectroscopy at 1 V bias in the dark has suggested the effective lifetime and global mobilities for P3HT: PCBM as 0.267 ms and 1.17 × 10⁻³ cm²/V-S and for P3HT: PCBM:Cu₂S NCs (20 wt%) systems as 0.156 ms and 2.02 × 10⁻³ cm²/V-S respectively. Based on observed photoluminescence quenching, calculated effective lifetime and global mobility, we have tried to explain the possible reason for improvement in the efficiency with the very well dispersion of Cu₂S NCs into the P3HT: PCBM matrix.     

Improving charge transport and suppressing charge recombination in small molecule ternary solar cells via incorporating Bis-PC71BM as a cascade material

The development of small molecule organic solar cells (SMOSCs) has attracted considerable attention and achieved comparable power conversion efficiency (PCE) with polymer solar cells. Here, we demonstrate a bulk heterojunction (BHJ) small molecular solar cell with PCE of 5.31% by incorporating Bisadduct of phenyl-C₇₁-butyric acid methyl ester (Bis-PC₇₁BM) as an additional acceptor material into the host binary blend of 2-[4-(N-butyl-N-phenylamino)-2,6-dihydroxyphenyl]-4-[(4-(N-butyl-N-phenylamino)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate (SQ-BP): [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM). The short circuit current (J_SC) and the fill factor (FF) of ternary SMOSCs are improved by decreasing the carrier recombination loss, increasing exciton dissociation and enhancing the carrier transport. The transient photovoltage (TPV) measurement indicates that the gradient HOMO energy alignment suppresses the charge recombination and leads to the increased charge carrier lifetime in ternary SMOSCs. As a result, the PCE of ternary devices with 5 wt% Bis-PC₇₁BM is about 20% greater than that of SQ-BP: PC₇₁BM based binary SMOSCs.     

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.     

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.     

Fullerene‐Free Polymer Solar Cells with Open‐Circuit Voltage above 1.2 V: Tuning Phase Separation Behavior with Oligomer to Replace Polymer Acceptor

This study has proposed to use a well‐defined oligomer F4TBT4 to replace its analogue polymer as electron acceptor toward tuning the phase separation behavior and enhancing the photovoltaic performance of all‐polymer solar cells. It has been disclosed that the oligomer acceptor favors to construct pure and large‐scale phase separation in the polymer:oligomer blend film in contrast to the polymer:polymer blend film. This gets benefit from the well‐defined structure and short rigid conformation of the oligomer that endows it aggregation capability and avoids possible entanglement with the polymer donor chains. The charge recombination is to some extent suppressed and charge extraction is also improved. Finally, the P3HT:F4TBT4 solar cells not only output a high V_OC above 1.2 V, but also achieve a power conversion efficiency of 4.12%, which is two times higher than the P3HT:PFTBT solar cells and is comparable to the P3HT:PCBM solar cells. The strategy of constructing optimum phase separation with oligomer to replace polymer opens up new prospect for the further improvement of the all‐polymer solar cells.     

Stabilization of poly(3-hexylthiophene)/PCBM morphology by hydroxyl group end-functionalized P3HT and its application to polymer solar cells

A new concept to stabilize the morphology of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) blend through H-bond formation by using a hydroxyl group end-functionalized P3HT (HOC-P3HT-COH) as a compatibilizer is presented. Domain size of the PCBM crystals in the annealed P3HT/PCBM film is diminished with addition of HOC-P3HT-COH. Surface roughness of the P3HT/PCBM film also becomes smoother with addition of HOC-P3HT-COH. Thermal stability of solar cell device is improved significantly through the H-bond formation between HOC-P3HT-COH and PCBM. A high performance and thermal stable polymer solar cell with 4.06% power conversion efficiency under AM1.5G irradiation is fabricated with 5% HOC-P3HT-COH in P3HT/PCBM layer.     

P3HT based solution-processed pseudo bi-layer organic solar cell with enhanced performance

We present a solution-processed pseudo bi-layer organic solar cell with poly(3-hexyl thiophene) (P3HT) as donor and indene-C₆₀ bisadduct (ICBA) as acceptor. The devices were fabricated by sequential processing of the active components followed by a thermal annealing treatment. An efficiency of 5.9% was achieved under AM 1.5G irradiation (1000 W/m²). The obtained efficiency is attributed to an enhanced nanomorphology that arises from the inter-diffusion of the ICBA molecules into a layer of pre-organised polymer (P3HT) and also due to the subsequent crystallisation of the ICBA molecules. These processes facilitate efficient charge generation and extraction. Time of flight-secondary ion mass spectroscopy (TOF-SIMS) depth profiling was carried out for different thermal annealing treatments of these pseudo bi-layer devices, which reveals full inter-diffusion of ICBA into the polymer P3HT. Photo-CELIV (charge extraction by linearly increasing voltage) studies elucidates that the thermal annealing imparts crystallinity to the fullerene phase which results in the improvement of charge carrier mobility.