Inverted Bulk Heterojunction Solar Cells Research Articles

Performance improvement of inverted bulk heterojunction solar cells by use of gold nanoparticles modified PEDOT: PSS as an anode buffer layer

Performance improvement of inverted bulk heterojunction solar cells by use of gold nanoparticles modified PEDOT: PSS as an anode buffer layer

Power losses in conventional and inverted non-polymeric donor:fullerene bulk heterojunction solar cells - The role of vertical phase separation in BQR:PC71BM blends

The performance of bulk heterojunction (BHJ) organic solar cells can be affected by a range of factors including the materials combination, processing solvent, post deposition annealing, and/or whether they are used in a conventional or inverted architecture. In this study we compared conventional and inverted BHJ solar cells composed of a non-polymeric donor (5 Z ,5′ Z )‐5,5'‐[(5‴,5‴''''‐{4,8‐bis[5‐(2‐ethylhexyl)‐4‐ n -hexylthiophen‐2‐yl]benzo[1,2‐b:4,5‐b']dithiophene‐2,6‐diyl}bis{3′,3'',3‴‐trihexyl‐[2,2':5′,2'':5'',2‴‐quaterthiophene]‐5‴,5‐diyl})bis(methanylylidene)]bis[3‐ n -hexyl‐2‐thioxothiazolidin‐4‐one] (BQR) and [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) as the acceptor. It was found that the conventional device structure had power conversion efficiencies nearly two and a half times that of the inverted device, 9.0% versus 4.0%. Through a combination of Shockley equivalent circuit fitting, optical modelling, and light intensity dependent photocurrent measurements we identified that the origin of the power losses for the inverted architecture relative to the conventional device structure arose from a larger component of bimolecular recombination. Neutron reflectometry measurements showed that the origin of the larger bimolecular recombination losses for the inverted device was due to the PC 71 BM phase separating, with a PC 71 BM rich layer located near the anode reducing the hole extraction efficiency. • BQR and PC 71 BM conventional and inverted BHJ solar cells are compared. • The conventional device structure was two and a half times more efficient. • Increased bimolecular recombination identified as the origin of the power losses for the inverted architecture. • A PC 71 BM rich layer near the anode reduced the hole extraction efficiency in inverted device.

Water‐Soluble Organic Dyes as Efficient Anode Interlayer Materials for PEDOT:PSS‐Free Inverted Bulk Heterojunction Solar Cells

Solution‐processed inverted organic solar cells (OSCs) generally use poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate (PEDOT:PSS) as hole selective anode interlayer (AIL). However, the acidic nature of PEDOT:PSS considerably accelerates the degradation dynamics of OSCs, which shortens the durability of these low‐cost photovoltaic devices. Small organic molecules are attracting growing interest as alternative AIL materials, but their solubility limited to toxic organic solvents hinders the production of environmentally friendly OSCs. Herein, the first inverted OSCs employing non‐PEDOT:PSS solution‐processed top small organic molecule AILs deposited from aqueous solution are reported. The investigated water‐soluble spirobifluorene (SBF) derivatives 1 and 2 show hole mobility (≈4 × 10−3 cm2 V−1 S−1) higher than PEDOT:PSS. Because of their nonacidic nature, the interlayers formed with derivatives 1 or 2 considerably delay the degradation of the top metal electrode compared to OSCs employing PEDOT:PSS interlayers. The PEDOT:PSS‐free OSC devices with inverted configuration with the water‐soluble SBF derivatives as AIL produce power conversion efficiencies above 5% with PTB7‐Th:ITIC active layers and above 8% with PBDB‐T‐2Cl:Y6 active layers, respectively, with an enhancement up to 28% compared to OSCs employing PEDOT:PSS. These results correspond to the highest reported values for PEDOT:PSS‐free small‐molecule inverted OSCs deposited from an aqueous solution.

A facile approach for enhancing device performance of excitonic solar cells with an innovative SnO2/TCNE electron transport layer

The spread of the Internet of things created the need for huge amounts of off-grid energy sources for tens of billions of electronic devices with low power consumption for indoor applications. Excitonic solar cells may provide a better solution as compared with other solar cells due to their advantages such as low-cost, large-area module, and fabrication by solution printing and coating techniques. However, the organic solar cells (OSCs) or perovskite solar cells (PSCs) should have a low energy loss, suitable absorption spectrum, and minimum trap mediated charge recombination for its proper commercialization. Interface engineering is one of the ways of achieving better performance for these devices. Herein, we report a facile and effective strategy for interfacial modulation to achieve the improved performance of inverted bulk heterojunction (BHJ) solar cells with a solution-processable SnO2 electron transport layer (ETL) modified by an organic small molecule tetracyanoethylene (TCNE). We can say that a few nano-meter thin film of TCNE supports to reduce the energy barrier of SnO2, resulting in the efficient extraction and transport of negative charge carriers toward the cathode. The TCNE also passivates the surface defects of SnO2 and hence decreases the charge recombination rate for iOSCs. Furthermore, it brings the better interfacial contact between SnO2 and BHJ blend in which the polymer PTB7 and fullerene PC71BM are the donor and acceptor, respectively, showing average power conversion efficiencies of ∼4.58%, ∼4.98%, and ∼2.95% with varying concentrations of TCNE (0.5 mg/ml, 1 mg/ml, and 2 mg/ml), respectively, in methanol, on top of SnO2, as ETLs, which are comparably better than that of pristine SnO2 (3.28%). We assumed that the TCNE modified SnO2 method is a potent and easy way to get efficient inverted BHJ solar cells with higher efficiency and may also be an appropriate alternative for other organic semiconducting devices where an ETL is required, such as organic light-emitting diodes and PSCs.

Performance of Air-Stable Cs<sub>2</sub>SnI<sub>6</sub> Perovskite as Electron Transport Layer in Inverted Bulk Heterojunction Solar Cell

In recent years, perovskite material has been extensively studied due to its unique physical properties and promising application in the third generation of solar cells. In particular, Sn-based perovskite has been considered to replace Pb-based perovskite because of the toxic effects and it can lead to other serious problems related to the environment. Cs2SnI6 perovskite has been known to be synthesized in a simple chemical process and it can be produced on a large scale. Moreover, this material is also oxygen and moisture stable due to the high oxidation state of tin. In this study, we synthesize air-stable Cs2SnI6 perovskite by the use of the wet chemical process at room temperature. Next, we attempt to fabricate the inverted bulk heterojunction solar cells incorporated Cs2SnI6 as electron transport layer in the configuration of ITO/ZnO/Cs2SnI6/P3HT:PCBM/PEDOT:PSS/Ag to improve device performance. The Cs2SnI6 perovskite shows an Fm-3m space group with a cubic lattice parameter of 11.62Å confirmed by X-Ray Diffraction (XRD) measurement, while UV-Vis measurement indicates this type of perovskite has direct band gap ~3.1 eV. The fabricated solar cell device reveals the enhancement in current density at short circuit condition (Jsc) from 64.69 mA/cm2 to 77.02 mA/cm2 with the addition of 2.25 mg/ml Cs2SnI6 along with the enhancement of power conversion efficiency (PCE) from 7.05% to 9.75% as characterized by J-V measurement. In our case, the voltage at open circuit condition (Voc) of the device does not perform significant improvement. Besides, it is found that the solar cell devices are quite stable even after exposure in the air for six weeks after fabrication, as indicated by PCE performance.

A Cross-Linkable Electron-Transport Layer Based on a Fullerene-Benzoxazine Derivative for Inverted Polymer Solar Cells.

The synthesis, optoelectronic characterization and device properties of a cross-linkable fullerene derivative, [6,6]-phenyl-C61 -butyric benzoxazine ester (PCBB) is reported. PCBB shows all the basic photophysical and electrochemical properties of the parent compound [6,6]-phenyl-C61 -butyric methyl ester (PCBM). Thermal cross-linking of the benzoxazine moiety in PCBB resulted in the formation of cross-linked, solvent resistive adhesive films (C-PCBB). Atomic force microscopy (AFM) and optical microscopic studies showed dramatic reduction in the roughness and aggregation behaviour of P3HT-PCBM polymer blend film upon incorporation of C-PCBB interlayer. An inverted bulk heterojunction solar cell based on the configuration ITO/ZnO/C-PCBB/P3HT-PCBM/V2 O5 /Ag achieved 4.27 % power conversion efficiency (PCE) compared to the reference device ITO/ZnO/P3HT-PCBM/V2 O5 /Ag (PCE=3.28 %). This 25 % increase in the efficiency is due to the positive effects of C-PCBB on P3HT/C-PCBB and PCBM/C-PCBB heterojunctions.

Graphene oxide-doped PEDOT:PSS as hole transport layer in inverted bulk heterojunction solar cell

Transparent poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) is widely used hole conducting material in optoelectronic devices. Secondary doping of PEDOT:PSS enables the tunability of its electronic properties. In this work, graphene oxide (GO) was used as a secondary dopant for PEDOT:PSS and the doped materials (composites) were tested for their efficiency as hole transport material in inverted bulk heterojunction (BHJ) solar cell. The composites were studied to unveil the effects of Coulombic interaction between GO and PEDOT:PSS where we note some segregation of PEDOT phase. We found that the GO majorly interacts with PSS through oxygeneous functional groups which promote the detachment of PEDOT from PSS and segregation of PEDOT. Electrochemical properties with and without illumination revealed some photo-induced changes to surface of the samples. Device performances showed about 2.2% efficiency enhancement when GO doping level was 0.25 (v:v) when compared to that of pristine PEDOT:PSS.

Effect of an Al-doped ZnO electron transport layer on the efficiency of inverted bulk heterojunction solar cells

Doping is a widely-implemented strategy for enhancing the inherent electrical properties of metal oxide charge transport layers in photovoltaic devices because higher conductivity of electron transport layer (ETL) can increment the photocurrent by reducing the series resistance. To improve the conductivity of ETL, in this study we doped the ZnO layer with aluminum (Al), then investigated the influence of AZO on the performance of inverted bulk heterojunction (BHJ) polymer solar cells based on 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):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM). The measured conductivity of AZO was ~10−3 S/cm, which was two orders of magnitude higher than that of intrinsic ZnO (~10−5 S/cm). By decreasing the series resistance (Rs) in a device with an AZO layer, the short circuit current (Jsc) increased significantly from 15.663 mA/cm2 to 17.040 mA/cm2. As a result, the device with AZO exhibited an enhanced power conversion efficiency (PCE) of 8.984%.

Inkjet printing of zinc oxide and P3HT:ICBA in ambient conditions for inverted bulk heterojunction solar cells

The third-generation organic solar cells have the benefits of low-cost, solution processability, low energy payback time, environment friendliness and appropriateness for mass production via large area printing techniques. The electron transport layer improves the ohmic contact between the cathode and the photoactive layer. In this research article, we demonstrate a drop on demand inkjet printing of the zinc oxide electron transport layer and the photoactive layer blend consisting of a donor poly(3-hexylthiophene-2,5-diyl) (P3HT) and acceptor Indene C-60 bisadduct (ICBA) of an organic solar cell. Consequently, all inkjet printed solar cell achieved a power conversion efficiency of 4.7% under ambient conditions. We highlighted that drop spacing, substrate temperature and the waveform parameters such as voltage and time are the important parameters for printing. Moreover, we observed a difference in morphology between spin coated and inkjet printed devices as characterized via UV–Visible absorbance, atomic force microscopy and scanning electron microscopy. Furthermore, we applied device characterization from impedance spectroscopy to elucidate the difference in device performance. Particularly, the effective lifetime and global mobilities calculated from the impedance spectroscopy Nyquist plots at 1 Voltage bias in the dark for all inkjet-printed devices are 0.0374 μs and 57.5 × 10−3 cm2/V–S respectively, and for all, spin-coated devices are 0.0628 μs and 115.3 × 10−3 cm2/V–S respectively.

Roles of Interfacial Tension in Regulating Internal Organization of Low Bandgap Polymer Bulk Heterojunction Solar Cells by Polymer Additives

AbstractThe role of a tertiary polymer‐based additive is investigated in increasing the efficiency of inverted low bandgap polymer:fullerene bulk heterojunction (BHJ) solar cells. Charge separation in polymer BHJ solar cells relies on the phase separation between electron accepting fullerene derivatives and photoactive polymers. Proper distribution of individual phases of suitable crystallinities within the active layer is a key factor for efficient charge transport/extraction and high photovoltaic performance. Here, it is demonstrated that the minor addition of a tertiary amorphous polymer, polystyrene (PS), with optimized molecular weights can increase the overall photovoltaic efficiency of poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]:phenyl‐C61butyric acid methyl ester (PCDTBT:PCBM) inverted BHJ solar cells, through the interfacial‐tension‐driven increase in crystallinities of photoactive phases and redistribution of PCBM molecules away from the top hole‐collecting anode interface. Complementary studies correlating polymer interfacial tension, blend internal structure, charge transport, and photovoltaic characteristics show that tertiary, high molecular‐weight polymers can serve as effective additives for improving the performance of low bandgap polymer solar cells.

Effect of the Molecular Configuration of Perylene Diimide Acceptors on Charge Transfer and Device Performance

Three perylene diimide (PDI)-based small molecules, T2-SePDI2, T3B-SePDI3, and T4B-SePDI4, with different molecular configurations are synthesized. Due to a large steric hindrance, the molecular configuration of T3B-SePDI3 is the most distorted, followed by T4B-SePDI4, while T2-SePDI2 shows the smallest steric hindrance. Inverted bulk heterojunction solar cells based on T3B-SePDI3 and PBDB-T show the highest power conversion efficiency (PCE) of 5.82% with an open-circuit voltage of 0.98 V, a high short-circuit current density of 10.52 mA/cm2, and a fill factor of 56.31%. The PCEs of the T2-SePDI2- and T4B-SePDI4-based devices are 4.10% and 5.10%, respectively. The results demonstrate that the molecular configuration of the PDI-based small molecule acceptor is critical and that increasing the steric hindrance is helpful in suppressing aggregation and improving device performance.

Increasing the fill factor of inverted polymer bulk heterojunction solar cells by doping PVP modified NaYF4 nanoparticles

ABSTRACTNaYF4 nanoparticles (NPs), synthesized by a facile solvothermal approach using polyvinylpyrrolidone (PVP) as a surfactant, were doped into P3HT:PCBM blend to fabricate inverted polymer bulk heterojunction solar cells. The results showed that the fill factor (FF) of the device was greatly enhanced by doping NaYF4 NPs/PVP composites into the active layer. There were evidences that PVP can form charge transfer (CT) complexes with PCBM. And the CT complexes, formed by PCBM and PVP carried by NaYF4 NPs, might make contributions to the phase separation of the active layer (P3HT:PCBM blend), which is vital to the FF of the devices.

New 1,7‐Disubstituted Perylenediimides as Molecular Acceptors for Organic Solar Cells

Two perylenediimides (PDIs), PT and PT2, functionalized with thiophene heterocycles on the 1,7‐positions of the bay region were synthesized and utilized in inverted bulk‐heterojunction (BHJ) polymer solar cells as acceptors in the photoactive layer. The synthesized PDIs were thermally stable up to 453°C and soluble in chlorinated solvents such as dichloromethane and chloroform. PT or PT2 was blended with poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]bithiophene‐co‐3‐fluorothieno[3,4‐b]thiophene‐2‐carboxylate] (PCE‐10) as a donor material. The inverted BHJ solar cell fabricated using the PCE‐10:PT photoactive layer showed the highest power conversion efficiency (PCE) of 2.06%. Film morphology studies of PCE‐10:PDI blends revealed a relationship between the film nanostructure and device performance.

Efficient Organic Photovoltaics with Improved Charge Extraction and High Short-Circuit Current

Exciton generation, dissociation, free carrier transport, and charge extraction play an important role in the short-circuit current (Jsc) and power conversion efficiency of an organic bulk heterojunction (BHJ) solar cell (SC). Here we study the impact of band offset at the interfacial layer and the morphology of active layer on the extraction of free carriers. The effects are evaluated on an inverted BHJ SC using zinc oxide (ZnO) as a buffer layer, prepared via two different methods: ZnO nanoparticle dispersed in mixed solvents (ZnO A) and sol–gel method (ZnO B). The device with ZnO A buffer layer improves the charge extraction and Jsc. The improvement is due to the better band offset and morphology of the blend near the ZnO A/active layer interface. Further, the numerical analysis of current–voltage characteristics illustrates that the morphology at the ZnO A/active layer interface has a more dominant role in improving the performance of the organic photovoltaic than the band offset.

Luminescent cathode buffer layer for enhanced power conversion efficiency and stability of bulk-heterojunction solar cells

We report high photovoltaic efficiency of over 9% in solution-processed, small-molecule (SPSM) 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c]1,2,5]thiadiazole) p-DTS(FBTTh2)2:[6-6]-phenyl C70 butyric acid methyl ester (PC70BM) blend based inverted BHJ solar cell by incorporating luminescent zinc oxide doped with sodium (ZnO:Na) quantum dots (QD) (l-ZnO) as a cathode buffer layer (CBL) in inverted bulk-heterojunction (BHJ) solar cells for the first time. The l-ZnO absorbs ultraviolet (UV) light and down-converts it to visible light. The l-ZnO layer's emission overlaps significantly with the absorption of p-DTS(FBTTh2)2, leading to an enhanced absorption by p-DTS(FBTTh2)2. This resulted in a significant enhancement of photo-current from 15.4 to 17.27 mA/cm2 and efficiency from 8% to 9.2% for ZnO and l-ZnO based devices, respectively. This is among one of the highest efficiency values reported so far in the case of SPSM based single junction BHJ solar cells. The luminescent ZnO layer also protects the active layer from UV-induced degradation as solar cells show high stability under constant solar light illumination retaining more than 90% (∼28 h) of its initial efficiency, whereas BHJ solar cells without the luminescent ZnO layer degraded to ∼50% of its initial value under same conditions. Since ZnO is an essential part of inverted organic solar cells, the luminescent l-ZnO CBL has great potential in inverted organic solar cells.

Heterojunction Solar Cells: Remarkably High Conversion Efficiency of Inverted Bulk Heterojunction Solar Cells: From Ultrafast Laser Spectroscopy and Electron Microscopy to Device Fabrication and Optimization (Adv. Energy Mater. 11/2016)

Heterojunction Solar Cells: Remarkably High Conversion Efficiency of Inverted Bulk Heterojunction Solar Cells: From Ultrafast Laser Spectroscopy and Electron Microscopy to Device Fabrication and Optimization (Adv. Energy Mater. 11/2016)

Remarkably High Conversion Efficiency of Inverted Bulk Heterojunction Solar Cells: From Ultrafast Laser Spectroscopy and Electron Microscopy to Device Fabrication and Optimization

In organic donor–acceptor systems, ultrafast interfacial charge transfer (CT), charge separation (CS), and charge recombination (CR) are key determinants of the overall performance of photovoltaic devices. However, a profound understanding of these photophysical processes at device interfaces remains superficial, creating a major bottleneck that circumvents advancements and the optimization of these solar cells. Here, results from time‐resolved laser spectroscopy and high‐resolution electron microscopy are examined to provide the fundamental information necessary to fabricate and optimize organic solar cell devices. In real time, CT and CS are monitored at the interface between three fullerene acceptors (FAs) (PC71BM, PC61BM, and IC60BA) and the PTB7‐Th donor polymer. Femtosecond transient absorption (fs‐TA) data demonstrates that photoinduced electron transfer from the PTB7‐Th polymer to each FA occurs on the sub‐picosecond time scale, leading to the formation of long‐lived radical ions. It is also found that the power conversion efficiency improves from 2% in IC60BA‐based solar cells to >9% in PC71BM‐based devices, in support of our time‐resolved results. The insights reported in this manuscript provide a clear understanding of the key variables involved at the device interface, paving the way for the exploitation of efficient CS and subsequently improving the photoconversion efficiency.

Syntheses, Charge Separation, and Inverted Bulk Heterojunction Solar Cell Application of Phenothiazine-Fullerene Dyads.

A series of phenothiazine-fulleropyrrolidine (PTZ-C60) dyads having fullerene either at the C-3 aromatic ring position or at the N-position of phenothiazine macrocycle were newly synthesized and characterized. Photoinduced electron transfer leading to PTZ(•+)-C60(•-) charge-separated species was established from studies involving femtosecond transient absorption spectroscopy. Because of the close proximity of the donor and acceptor entities, the C-3 ring substituted PTZ-C60 dyads revealed faster charge separation and charge recombination processes than that observed in the dyad functionalized through the N-position. Next, inverted organic bulk heterojunction (BHJ) solar cells were constructed using the dyads in place of traditionally used [6,6]-phenyl-C61- butyric acid methyl ester (PCBM) and an additional electron donor material poly(3-hexylthiophene) (P3HT). The performance of the C-3 ring substituted PTZ-C60 dyad having a polyethylene glycol substituent produced a power conversion efficiency of 3.5% under inverted bulk heterojunction (BHJ) configuration. This was attributed to optimal BHJ morphology between the polymer and the dyad, which was further promoted by the efficient intramolecular charge separation and relatively slow charge recombination promoted by the dyad within the BHJ structure. The present finding demonstrate PTZ-C60 dyads as being good prospective materials for building organic photovoltaic devices.

Polypyridyl complexes as electron transporting materials for inverted bulk heterojunction solar cells: the metal center effect

The fundamental science behind the design of organic photovoltaic (OPV) cells lies in the formation of energy level gradients for efficient charge separation and collection.

Squaraine based solution processed inverted bulk heterojunction solar cells processed in air.

Inverted bulk heterojunction solar cells based on low temperature solution processed squaraine (SQ) and [6,6]-phenyl C71 butyric acid methyl-ester (PC71BM) with varying blend ratios were made in air. An optimized bulk heterojunction device of SQ and PC71BM (with a blend ratio of 1 : 6) showed a power conversion efficiency (PCE) of 2.45% with an incident photon to current conversion efficiency of 65% at 680 nm and a spectral window extending to the NIR region. The devices also showed an enhanced PCE value of 4.12% upon continuous illumination from an AM1.5G light source of intensity 1 Sun. The intensity dependent photocurrent studies showed a monomolecular recombination mechanism in the photovoltaic device performance. The device stored in air showed reasonable stability for a period of one month.