Introducing Double Cathode Modification Layers into Organic Solar Cells

By Samudrapom DamReviewed by Susha Cheriyedath, M.Sc.Oct 20 2022

A paper recently published in the journal Energies demonstrated the feasibility of using double cathode modification layers to fabricate organic solar cells (OSCs) with high stability and efficiency.

Study: Double Cathode Modification Improves Charge Transport and Stability of Organic Solar Cells. Image Credit: nevodka/Shutterstock.com

Background

Organic solar cells (OSCs) are considered a suitable candidate to access clean energy, specifically solar energy. Significant advancements have been made in the field of OSCs in recent years owing to the development of new synthetic materials, morphology control of active layers, and device structure optimization.

Moreover, the adoption of interface modification layers has enabled the development of highly stable and efficient OSCs by improving the electrode’s charge extraction ability and regulating the energy barrier between organic active layers and the electrode.

Poly(3,4-(ethylenedioxy)thiophene):poly(styrenesulfonate) (PEDOT:PSS) is used most extensively as an anode buffer layer material during OSC fabrication owing to its enhanced transparency in the spectral response range and superior hole transport ability. Both organic and inorganic materials are used as modification materials for the cathode buffer layer.

Inorganic materials, including metal oxides such as titanium oxide (TiO_X) and zinc oxide (ZnO), lithium fluoride (LiF), and organic materials such as perylene diimide derivatives (PDIN-o), poly[(9,9-bis(3'-(N, N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9 –dioctylfluorene)] (PFN), and polyethylenimine ethoxylated (PEIE), are typically used as cathode buffer layer materials.

Ensuring long-term stability and improving power conversion efficiency (PCE) are the most significant challenges associated with developing OSCs. The OSC lifetime can be improved by reducing the extent of damage caused by oxygen and moisture. Different methods, such as device encapsulation and synthesis of hydrophobic organic materials, have been considered to improve long-term device stability.

Among them, the electrode modification layer regulation is considered a more suitable approach to prevent the penetration of oxygen molecules and moisture into the device that affects both electrode and the active layer. Thus, this method can play a crucial role in improving the device's stability and efficiency.

The Study

In this study, researchers innovatively introduced a double cathode modification layer to improve the stability and charge transport of OSCs. They used a tin(IV) oxide (SnO₂)/ZnO double cathode modification layer into a non-fullerene OSC based on 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (IT-4F) as acceptor and poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-chloro)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c’]dithiophene-4,8-dione)] (PM7) as donor (PM7:IT-4F) and investigated the effects of ZnO/SnO₂ film on the charge carrier transfer in the solar device.

Although the inorganic double cathode modification layer was previously used in solar cells based on other materials, such as perovskites, such a modification layer was introduced in PM7:IT-4F-based OSCs for the first time.

o-Xylene (OX) was used as the active solvent layer, while molybdenum trioxide (MoO₃) and ZnO/SnO₂ were used to prepare the anode buffer layer and electron-transport layer, respectively. The ZnO was synthesized using propane-1,2-diol, 2-aminoethanol, and zinc acetate dihydrate through the sol-gel method.

Initially, PM7 was mixed with IT-4F in a solution bottle in a 1.25:1 mass ratio, and dichloromethane was added to the resultant mixture. The bottle was then left in a glove box with high-purity nitrogen to dry the mixture thoroughly. Subsequently, the acceptor and donor materials were dissolved in OX by stirring them for six h at 60 ^oC.

OSCs were fabricated using three types of cathode modification layers, including SnO₂, ZnO, and ZnO/SnO₂. They featured an inverted structure consisting of indium tin oxide (ITO)/buffer layer/PM7:IT-4F/MoO₃/silver.

All devices were synthesized on cleaned ITO-coated glass substrates. The ITO-coated substrates were dried using high-purity nitrogen after cleaning and then subjected to ultraviolet (UV)-ozone treatment for eight min.

Subsequently, the buffer layer was spin-coated as the cathode modification layer on the treated ITO-coated substrates using the sol-gel method. The structure with SnO₂/ZnO layer was annealed for 40 min at 180 ^oC, while structures with SnO₂ and ZnO were annealed for 60 min at 200 ^oC and 30 min at 150 ^oC, respectively.

A PM7:IT-4F layer was spin-coated on the fabricated ITO/cathode modification layer films and dried for 30 min to synthesize the dry active layer film. The as-prepared ITO/cathode modification layer/active layer was then placed in a thermal evaporative coating instrument and vacuumed to 1.5 × 10⁻⁶ Torr. Eventually, the MoO₃ and silver were deposited successively on the fabricated structure as an anode buffer layer and anode, respectively.

Researchers measured the thickness of each layer in the OSCs, recorded the PM7:IT-4F film UV-visible absorption spectra with various electronic transport layers such as ZnO and SnO₂/ZnO, obtained the current density-voltage (J-V) characteristic curves and external quantum efficiency curves of every fabricated device, and measured the photo-generated charge extraction by linearly increasing voltage (photo-CELIV) and transient photocurrent/transient photovoltage (TPC/TPV) to characterize the fabricated devices.

Observations

Inversion OSCs based on the PM7:IT-4F system with high stability and efficiency were fabricated successfully using the ZnO/SnO₂ film as a double cathode modification layer. Devices with ZnO/SnO₂ double cathode modified layer demonstrated the advantages of both single SnO₂ and ZnO films.

The ZnO cover on the SnO₂ film effectively passivated the SnO₂ surface defects, which blocked the holes and facilitated a more efficient transport of electrons. The SnO₂/ZnO double modification layer improved the OSC PCE to 12.91% with a 70% fill factor, 0.92 V open circuit voltage, and 20.04 mA/cm² short-circuit current density compared to the single modification layer.

The reduction in the energy barrier between the active layer and electrode and decreasing trap near the interface layer resulted in efficient charge collection and transfer and suppressed charge recombination near the active layer.

A denser SnO₂/ZnO modification layer prevented the infiltration of the active material into the electrode and effectively reduced the penetration of oxygen and water into the active layer/modification layer interface, which decreased the rate of dark degradation in both air and nitrogen atmosphere by 17% and 7%, respectively.

To summarize, the findings of this study effectively demonstrated the feasibility of using a double cathode modification layer to fabricate OSCs with high stability and efficiency. Moreover, the study also elaborates on the mechanisms that can improve the device stability through cathode modification, which can provide insights for future OSC interfacial modification studies. 

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