Planar Heterojunction Organic Solar Cells Research Articles

Suppressing Bimolecular Charge Recombination and Energetic Disorder with Planar Heterojunction Active Layer Enables 18.1% Efficiency Binary Organic Solar Cells

The performance of organic solar cells has been dramatically enhanced due to the use of bulk heterojunction active layer structure. However, blending donor and acceptor in the bulk volume results in a large extent of bimolecular charge recombination and increasing energetic disorder. Herein, we fabricated organic solar cells with polymer donor PM6 and nonfullerene acceptor N3 in bilayer structure to investigate the impact of planar heterojunction configurations on charge recombination and energetic landscapes. We find superior crystallinity features in the bilayer composed of pure donor and acceptor layers. The bimolecular charge recombination and energetic disorder are effectively suppressed compared with the bulk heterojunction counterparts. Thus, a power conversion efficiency as high as 18.1% (17.8% averaged) is achieved, which stands among the top values of planar heterojunction organic solar cells. In addition, improved device shelf stability is observed because of the fewer donor–acceptor interfaces in the active layer. Our results suggest that a planar heterojunction structure could efficiently suppress the bimolecular charge recombination and energetic disorder, providing an alternative pathway for developing solution-processed organic solar cells.

Nonhalogenated Solution‐Processed Donor‐Dispersed Planar Heterojunction Organic Solar Cells with Enhanced Homogeneity in Vertical Phase Separation

Realization of state‐of‐the art efficiencies in organic photovoltaics (OPV) generally relies on using toxic halogenated solution processing to arrive at the desired nanomorphology and optoelectronic responses, whereas the photovoltaic performance in nonhalogenated solution (NHS)‐based OPVs is yet nonsatisfactory, mainly related to the difficulty of morphological control. Herein, a conceptual approach of donor‐dispersed planar heterojunction (DD‐PHJ) for improving the regulation of phase morphology and photovoltaic behaviors in NHS‐processed OPVs is proposed, afforded by dispersing an ordered liquid crystal guest donor BTR‐Cl into the nonfullerene acceptor host with sequential film deposition. The combined investigation shows that the inclusion of BTR‐Cl plays a regulatory role in enhancing the crystallization, intermolecular donor/acceptor miscibility, and homogeneity in the donor–acceptor phase separation along vertical direction, which is conducive to improved charge transfer and reduced photovoltage loss. Of importance, the described DD‐PHJ approach is applicable to representative OPV material systems, leading to a champion efficiency of 18.21% in devices prepared with NHS. This work provides a promising prospect toward high‐efficiency and green solution‐processed OPV devices.

Ternary Planar Heterojunction Organic Solar Cells Based on the Ternary Active Layers: α-6T/AlPcCl/C60

Ternary planar heterojunction organic solar cells (PHJ-OPVs) were fabricated using three organic small molecules, alpha-sexithiophene (α-6T), aluminum phthalocyanine chloride (AlPcCl) and fullerene (C60). These molecules can be easily sublimated under a vacuum; they have complementary optical absorption spectra and their energy band structure alignment is favorable for electronic charge transfers. Moreover, α-6T and AlPcCl have almost the same HOMO, which is desirable to avoid any decrease in open circuit voltage. The AlPcCl intercalated layer bridges the energy levels of the electron donor, α-6T, and the electron acceptor, C60, which facilitates charge transport through the energy cascade effect. Moreover, the charge carrier mobility measurements of AlPcCl, using the space charge limited current method, demonstrated that it iss ambipolar. All these properties combine to improve the power conversion efficiency (PCE) of PHJ-OPVs by moving from binary structures (α-6T/C60, α-6T/AlPcCl and AlPcCl/C60) to ternary ones (α-6T/AlPcCl/C60). We show, in this study, that both interfaces of the ternary PHJ-OPVs are efficient for carrier separation. After optimization of the different layer thickness, we show that, by comparing the optimum efficiencies of the binary PHJ-OPVs, the realization of ternary PHJ-OPVs, based on the active layers α-6T/AlPcCl/C60, using the following optimized method, allows us to achieve a PCE of 4.33%.

Investigation of the different possible energy band structure configurations for planar heterojunction organic solar cells

We present a study of performances of organic photovoltaic cells (OPVs) using a wide variety of small molecules. These OPVs use two or three molecules to form binary or ternary cells, i.e. consisting of two or three flat stacked layers. Different ternary OPVs designs for efficient light harvesting and energy band structure combinations are studied. Molecules have been chosen to study all possible energy band structure alignments in ternary configuration. We discuss various parameters limiting their efficiency, such as carrier mobility, organic layer morphology, energy band structure alignment. Special attention has been paid to understand the device behavior. It is shown that, if light absorption domains, carriers mobility, energy band structure alignment are decisive for achieving performing OPVs, the geometry of the molecules is also decisive. In ternary OPVs, the third organic layer, intercalated between the electron donor and the electron acceptor, must serve as a bridge in the energy levels of the outer layers, which permits an energetic cascade effect. For this effect to be positive it is necessary that this intermediate layer facilitates creation and transport of charge carriers, but also that it allows transport of energy, all leading to an improvement of the performance of the OPVs.

Strongly Reduced Non-Radiative Voltage Losses in Organic Solar Cells Prepared with Sequential Film Deposition.

With nearly 100% yields for mobile charge carriers in organic solar cells (OSCs), the relatively large photovoltage loss (ΔVoc) is a critical barrier limiting the power conversion efficiency of OSCs. Herein, we aim to improve the open-circuit voltage (Voc) in OSCs with non-fullerene acceptors via sequential film deposition (SD). We show that ΔVoc in planar heterojunction (PHJ) devices prepared by the SD method can be appreciably mitigated, leading increases in Voc to 80 mV with regard to the Voc of bulk heterojunction devices. In PHJ OSCs, the energy level of intermolecular charge-transfer states is found to increase with a decrease in the level of aggregation in the solid state. These properties explain the enhanced electroluminescent quantum efficiency and resultant suppression of the voltage losses induced by nonradiative charge recombination and interfacial charge transfer. This work provides a promising strategy for tackling the heavily discussed photovoltage loss in OSCs.

All‐Green Solvent‐Processed Planar Heterojunction Organic Solar Cells with Outstanding Power Conversion Efficiency of 16%

AbstractDevice engineering is an effective way to improve the photovoltaic performance of organic solar cells (OSCs). Currently, the widely used bulk heterojunction (BHJ) structure has problems such as material solubility limitations and the emerging pseudoplanar heterojunction (PPHJ) structure is also troubled by printing technology requirements. However, these issues can be solved by the reasonable application of traditional planar heterojunction (PHJ) structure. Herein, PM6:BO‐4F system is selected to prepare PHJ devices by combining sequential spin‐coating and orthogonal solvent strategy. In view of the good solubility of PM6 and BO‐4F in commonly used high‐boiling solvent chlorobenzene (CB) and green solvent tetrahydrofuran (THF), respectively, the PHJ devices are successfully prepared by using these two orthogonal solvents, achieving a power conversion efficiency (PCE) of 15.6%. On this basis, green nonhalogen reagent o‐xylene (O‐XY) is further used to process PM6. Due to the large polarity difference between O‐XY and THF, all‐green solvent‐processed PHJ devices are successfully fabricated and obtain an astonishing PCE of 16%. As far as it is known, it is the highest efficiency for PHJ OSCs. The results prove the huge research potential of PHJ structure and point out new direction for solving OSC materials compatibility, long‐term stability, and future commercial applications.

Overcoming incompatibility of donors and acceptors by constructing planar heterojunction organic solar cells

Nanoscale phase separation plays a critical role in achieving high photovoltaic performance in bulk-heterojunction organic solar cells, which is limited by solubility, crystallinity, and the compatibility of the donors and acceptors. However, these limits can be overcome by new applications of the traditional planar heterojunction structure. Herein, we demonstrate quasi-orthogonal solvents for a wide-band-gap donor (P2F-EHp) and a highly crystalline non-fullerene acceptor (M4–4F) to overcome the over-agglomeration and enabled the fabrication of a planar heterojunction device with a favorable power conversion efficiency of 14.2% (compared with 12.7% in bulk-heterojunction device), which is among one of the highest efficiencies of planar heterojunction devices to date. Detailed studies based on cross-sectional transmittance electron microscopy, grazing incidence wide-angle X-ray scattering at various incident angles, and resonance soft X-ray scattering measurements confirmed the formation of optimal nanoscale morphology. Further transient absorption measurements also proved that the exciton dissociation and free-carrier generation efficiency are not limited by the small mixed-phase area and large pure domains. These findings show insights into the intrinsic morphological changes and charge transfer process in bulk-heterojunction and planar heterojunction structures, providing a novel method for matching donors and acceptors with relatively poor compatibility. • Comparing absorption, morphology, and charge carrier dynamics of BHJ and PHJ devices. • Introducing an effective approach to construct OSCs using poorly compatible donors and acceptors. • Demonstrating the possibility of obviating the need for bulk-heterojunction structure.

Planar heterojunctions for reduced non-radiative open-circuit voltage loss and enhanced stability of organic solar cells

Planar heterojunction organic solar cells exhibit lower trap density, higher electroluminescence efficiency, smaller non-radiative open-circuit voltage loss and better stability than bulk heterojunction counterparts.

Triphenylamine/Tetracyanobutadiene-Based π-Conjugated Push-Pull Molecules End-Capped with Arene Platforms: Synthesis, Photophysics, and Photovoltaic Response.

π-Conjugated push-pull molecules based on triphenylamine and 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) have been functionalized with different terminal arene units. In solution, these highly TCBD-twisted systems showed a strong internal charge transfer band in the visible spectrum and no detectable photoluminescence (PL). Photophysical and theoretical investigations revealed very short singlet excited state deactivation time of ≈10 ps resulting from significant conformational changes of the TCBD-arene moiety upon photoexcitation, opening a pathway for non-radiative decay. The PL was recovered in vacuum-processed films or when the molecules were dispersed in a PMMA matrix leading to a significant increase of the excited state deactivation time. As shown by cyclic voltammetry, these molecules can act as electron donors compared to C60 . Hence, vacuum-processed planar heterojunction organic solar cells were fabricated leading to a maximum power conversion efficiency of ca. 1.9 % which decreases with the increase of the arene size.

On the contribution of fullerene to the current of planar heterojunction organic solar cells

Recently, significant progress in the field of organic photovoltaic cells was obtained by substituting new electron acceptor molecules to the fullerene, which was attributed to the fact that the fullerene absorption is quite small. Nevertheless, we demonstrate in the present work that, in the case of inverted cells, i.e. when the transparent bottom electrode is used as cathode, the contribution of fullerene to the Jsc short-circuit current of the cells, if not dominant, is not negligible; and that mainly in the short wavelength spectral range. The experimental results are confirmed by an optical simulation. Due to this significant contribution to Jsc, the light transmission of the transparent electrode towards the UV-part of the spectrum is crucial for inverse cell performances. When a transparent conductive electrode based on an alternative dielectric/metal/dielectric structure is substituted to ITO, such as ZnS/Ag/TiO2, the study allows to obtain promising results, although there is a loss of performance due to the decrease of transmission of TiO2 below 400 nm in wavelength.

Investigation of post-thermal annealing-induced enhancement in photovoltaic performance for squaraine-based organic solar cells

In this work, we have systematically investigated the post-thermal annealing-induced enhancement in photovoltaic performance of a 2,4-bis[4-(N, N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (DIBSQ)/C60 planar heterojunction (PHJ) organic solar cells (OSCs). An increased power conversion efficiency (PCE) of 3.28% has been realized from a DIBSQ/C60 device with thermal annealing at 100 °C for 4 min, which is about 33% enhancement compared with that of the as-cast device. The improvement of the device performance may be mainly ascribed to the crystallinity of the DIBSQ film with post-thermal annealing, which will change the DIBSQ donor and C60 acceptor interface from ^PHJ to hybrid planar-mixed heterojunction. This new donor-acceptor heterojunction structure will significantly improve the charge separation and charge collection efficiency, as well as the open circuit voltage (Voc) of the device, leading to an enhanced PCE. This work provides an effective strategy to improve the photovoltaic performance of SQ-based OSCs.

Effect of 4-biphenyl groups on the charge transport and photovoltaic properties of arylamine based push–pull systems

The synthesis of two N,N-bis(4-biphenylyl)aniline (BPA) based push–pull is reported herein. Evaluation as donor materials in all-small-molecule planar-heterojunction organic solar cells lead to impressive fill factor (>73%) and promising power conversion efficiencies.

The effect of the band structure on the Voc value of ternary planar heterojunction organic solar cells based on pentacene, boron subphthalocyanine chloride and different electron acceptors

Using three organic materials in the cascade configuration of organic photovoltaic cells (OPVs) broadens the absorption range of visible light, resulting in an increase in the short circuit current density (Jsc). Herein, we report for the first time the use of three organic molecules, pentacene, boron subphthalocyanine chloride (SubPc) and fullerene (C60). Upon comparison with the binary pentacene/C60 and SubPc/C60structures, the high Jsc value obtained for the ternary structure induces an increase in the OPV efficiency. This improvement is limited by the small open circuit voltage (Voc) value due to the low absolute value of the highest occupied molecular orbital of pentacene. Our experimental study confirmed that the Voc is ultimately limited by the energy levels of the outer layers in these cascade structures. Initial attempts to overcome this bottleneck were carried out using a variety of electron acceptors as an alternative to fullerene. However, increasing the Voc was detrimental to the current density, therefore the best OPVs remain those constructed using fullerene.

Origin of high open-circuit voltage in a planar heterojunction solar cell containing a non-fullerene acceptor

Vapor-deposited, planar heterojunction organic solar cells containing a periflanthene donor and either a fullerene or non-fullerene acceptor are investigated. A high VOC of 1.16 V is observed in devices containing the non-fullerene, pyrrolo[3,4-c]pyrrole-1,4-dione, 3,6-bis(4-chlorophenyl)-2,5-dihydro acceptor, whereas analogous devices containing C60 only result in a VOC of 0.8 V. The measured band energy levels of the two different acceptors do not readily explain the observed difference. Small-perturbation transient photovoltage and transient photocurrent measurements reveal that interfacial charge recombination is comparatively slower for the non-fullerene acceptor, resulting in relatively higher Voc values.

Efficient Charge Separation of Cold Charge-Transfer States in Organic Solar Cells Through Incoherent Hopping.

We demonstrate that efficient and nearly field-independent charge separation of electron-hole pairs in organic planar heterojunction solar cells can be described by an incoherent hopping mechanism. Using kinetic Monte Carlo simulations that include the effect of on-chain delocalization as well as entropic contributions, we simulate the dissociation of the charge-transfer state in polymer-fullerene bilayer solar cells. The model further explains experimental results of almost field independent charge separation in bilayers of molecular systems with fullerenes and provides important guidelines at the molecular level for maximizing the efficiencies of organic solar cells. Thus, utilizing coherent phenomena is not necessarily required for highly efficient charge separation in organic solar cells.

Understanding the effects of TCO work function on the performance of organic solar cells by numerical simulation

The influences of the work function of transparent conducting oxides (TCO) on the performance of organic solar cells, including open circuit voltage, conversion efficiency and fill factor, has been simulated. It is found that for non-ohmic contact the open circuit voltage and conversion efficiency increase monotonically with the TCO work function but remain constant for ohmic contact. The fill factor decreases and increases with the electrode work function when the electrode work function is below and above a critical value (4.2 eV for TCO and 4.5 eV for back-contact), respectively. The results of this simulation are significant in the choice of TCO contacts to optimize organic planar heterojunction solar cells.

Contorted Hexabenzocoronenes with Extended Heterocyclic Moieties Improve Visible-Light Absorption and Performance in Organic Solar Cells

The large band gaps of existing contorted hexabenzocoronene derivatives severely limit visible-light absorption, restricting the photocurrents generated by solar cells utilizing contorted hexabenzocoronene (cHBC). To decrease the band gap and improve the light-harvesting properties, we synthesized cHBC derivatives having extended heterocyclic moieties as peripheral substituents. Tetrabenzofuranyldibenzocoronene (cTBFDBC) and tetrabenzothienodibenzocoronene (cTBTDBC) both exhibit broader absorption of the solar spectrum compared to cHBC, with peak absorbances on the order of 105 cm–1 in the near-ultraviolet and in the visible. Planar-heterojunction organic solar cells comprising cTBFDBC or cTBTDBC as the donor and C70 as the acceptor surpass those having cHBC in photocurrent generation and power-conversion efficiency. Interestingly, devices containing cTBFDBC/C70 exhibit the highest photocurrents despite cTBTDBC having the smallest band gap of the three cHBC derivatives. X-ray reflectivity of the active la...

Thickness-dependence of S-shaped J–V curves of planar heterojunction organic solar cells containing NTCDA interlayer: Impedance–potential measurement and underlying mechanism

We present the thickness-dependence of the performance of planar heterojunction organic solar cells (OSCs) with 1,4,5,8-napthalene-tetracarboxylic-dianhydride (NTCDA) as interlayer between rubrene/C70. The S-shaped current–voltage (J–V) characteristic is observed in the optimization process of NTCDA thickness. With the increase of NTCDA thickness, device performance first increases then decreases, with the optimum thickness of 15nm. The observed thickness-dependence of S-kink is totally different from previous reports of BCP based OSCs. The anomalous J–V characteristics were investigated by using the impedance–potential technique. We find that larger impedance occurred in the device with thinner NTCDA interlayer, and the mechanism could be described by charge distribution between the interface of C70 and Al cathode. Finally, we propose an effective approach to eliminate the S-kink in J–V curves by postannealing processes.

Esterification of Indoline-Based Small-Molecule Donors for Efficient Co-evaporated Organic Photovoltaics

In this study, we attempted to develop a protocol for fabricating efficient solid-state organic photovoltaics based on materials used currently for dye-sensitized solar cells. Three typical indoline dyes, namely, D131, D102, and D149, were employed as electron donors in conjunction with C70 fullerene in solution-processed planar-heterojunction (PHJ) organic solar cells (OSCs). The PHJ cells based on these dyes exhibited similar external quantum efficiencies over the entire spectral range, resulting in identical short-circuit photocurrents. The open-circuit voltages of the PHJ cells were consistent with the highest occupied molecular orbital level of the corresponding indoline dye. The D102-based PHJ cell exhibited the highest power conversion efficiency, of up to 3.1%. The efficiency was limited by the light-harvesting capability of the solar cell, given that the short diffusion length (∼5 nm) of D102 limited the thickness of the active layer; the diffusion length was determined through an optical simulat...

Amorphous vs crystalline exciton blocking layers at the anode interface in planar and planar-mixed heterojunction organic solar cells

We compare the gain in power conversion efficiency (PCE) achieved by inserting either amorphous or crystalline exciton blocking layers at the anode interface for planar (PHJ) and planar-mixed heterojunction (PM-HJ) organic solar cells based on Tetraphenyldibenzoperiflanthene and fullerenes. For PHJ devices, there is a gain of more than 37% for both types of blocking layers, mainly due to an increase in photocurrent, indicating that this gain can be solely ascribed to the exciton blocking effect. A templating effect as proposed in literature for crystalline blocking layers cannot be affirmed. On the contrary, it is shown that there is a connection between the choice of acceptor (C60/C70) and the blocking effect on the anode side. Moreover, we can show that also for PM-HJ devices a remarkable efficiency enhancement is possible. The insertion of suitable blocking layers at the anode interface can alter the effective work function and thus the open-circuit voltage, leading to a maximum PCE of 5.8% in single junction cells.