Quantification of deep hole-trap filling by molecular p-doping: Dependence on the host material purity

The p-doping effect of the fluorinated fullerene C₆₀ F₃₆ doped into organic thin films of N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) of different purification grades is systematically investigated by photoemission spectroscopy. By reducing the molar doping ratio to MR = 2.9 × 10^?4, the Fermi-level shift upon doping is resolved in particular at very low doping concentrations. In comparison to four times sublimated MeO-TPD, 5 times more C₆₀F₃₆ molecules have to be doped into unpurified MeO-TPD films to shift the Fermi-level just above its intrinsic position. This finding is discussed in terms of a statistical model, showing that narrow deep hole-trap states are additionally present in the unpurified host material which are hindering an efficient generation of free charge carriers at molar doping ratios below MR = 0.002.     

Substrate effect on the interfacial electronic structure of thermally-evaporated CH3NH3PbI3 perovskite layer

The impact of substrate work function on the interfacial electronic structure of thermally-evaporated CH₃NH₃PbI₃ perovskite films on various substrates have been systematically investigated using in-situ ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). On substrates with work function lower than ∼4.43 eV, a Fermi level pinning effect of the lowest unoccupied molecular orbital (LUMO) is observed, resulting in the near zero electron extraction barrier for the CH₃NH₃PbI₃ perovskite solar cells. On the other hand, when substrates with high work function are used, even exceed the highest occupied molecular orbital (HOMO) of CH₃NH₃PbI₃, an almost constant hole extraction barrier of ∼0.88 eV is observed, indicating that the efficiency of hole extraction at these interfaces are low. In order to understand the low hole extraction efficiency at interfaces between CH₃NH₃PbI₃ and these high work function electrodes, the evolution of electronic structures at the interface between CH₃NH₃PbI₃ and MoO₃ is further investigated. The charge transfer and dipole formation between CH₃NH₃PbI₃ and MoO₃ are deduced from the UPS and XPS results, and the energy level alignment between CH₃NH₃PbI₃ and MoO₃ is discussed.     

Light management in PCPDTBT:PC70BM solar cells: A comparison of standard and inverted device structures

We compare standard and inverted bulk heterojunction solar cells composed of PCPDTBT:PC₇₀BM blends. Inverted devices comprising 100 nm thick active layers exhibited short circuit currents of 15 mA/cm², 10% larger than in corresponding standard devices. Modeling of the optical field distribution in the different device stacks proved that this enhancement originates from an increased absorption of incident light in the active layer. Internal quantum efficiencies (IQEs) were obtained from the direct comparison of experimentally derived and modeled currents for different layer thicknesses, yielding IQEs of ∼70% for a layer thickness of 100 nm. Simulations predict a significant increase of the light harvesting efficiency upon increasing the layer thickness to 270 nm. However, a continuous deterioration of the photovoltaic properties with layer thickness was measured for both device architectures, attributed to incomplete charge extraction. On the other hand, our optical modeling suggests that inverted devices based on PCPDTBT should be able to deliver high power conversion efficiencies (PCEs) of more than 7% provided that recombination losses can be reduced.     

Effect of processing additive 1,8-octanedithiol on the lifetime of PCPDTBT based Organic Photovoltaics

The effect that processing additives have upon the lifetime of PCPDTBT-based OPVs has been investigated. The results show conclusively that whilst ODT processing additive enhances the initial performance of PCPDTBT:PCBM OPVs, it is detrimental to their long term performance, when measured under light soaking at 1 Sun of irradiance. Results are shown for both encapsulated and non-encapsulated devices. Topographical and morphological measurements made using AFM and small- and wide-angle X-ray scattering of active layers show that there are greater morphological changes of devices fabricated with the 1,8-octanedithiol upon light soaking, revealing a relatively venerable morphology of the active layer processed with the additive, when subject to light soaking.     

Towards Simplifying the Device Structure of High‐Performance Perovskite Solar Cells

Perovskite solar cells (PSCs) are considered one of the most promising next‐generation examples of high‐tech photovoltaic energy converters, as they possess an unprecedented power conversion efficiency with low cost. A typical high‐performance PSC generally contains a perovskite active layer sandwiched between an electron‐transport layer (ETL) and a hole‐transport layer (HTL). The ETL and HTL contribute to the charge extraction in the PSC. However, these additional two layers complicate the manufacturing process and raise the cost. To extend this technology for commercialization, it is highly desired that the structure of PSCs is further simplified without sacrificing their photovoltaic performances. Thus, ETL‐free or/and HTL‐free PSCs are developed and attract more and more interest. Herein, the commonly used methods in reducing the defect density and optimizing the energy levels in conventional PSCs in order to simplify their structures are summarized. Then, the development of diverse ETL‐free or/and HTL‐free PSCs is discussed, with the PSCs classified, including their working principles, implemented technologies, remaining challenges, and future perspectives. The aim is to redirect the way toward low‐cost and high‐performance PSCs with the simplest possible architecture.     

Hole extraction layer utilizing well defined graphene oxide with multiple functionalities for high-performance bulk heterojunction solar cells

A simple method for synthesizing a series of graphene oxide with precise oxidation (pr-GO) (mild oxidation, moderate oxidation and severe oxidation) by strictly controlling pre-oxidation steps, oxidant content and oxidation time has been successfully developed. The well defined pr-GO as hole extraction layer (HEL) presented multiple functionalities, like modulation of work function, enhanced interfacial dipole, and excellent film-forming properties, which had significantly improved the efficiency and stability of organic solar cells. The P3HT:PC₆₁BM system device based on pr-GO-3 HEL, which possessing well defined electronic structure and moderate oxidation, exhibited an improved 3.74% in power conversion efficiency and better air-stability compared to that of other pr-GOs and conventional PEDOT:PSS based devices. The well defined electronic structure pr-GO (i.e., suitable work function, larger interfacial dipole, and high repeatability) will provide better understanding in utilizing pr-GO film as HEL in future solar cell applications.     

Energy level alignment at the porphyrin/cobaltocene interface: From transfer doping to cobalt intercalation

N-type doping of the organic semiconductor zinc tetraphenylporphyrin (ZnTPP) by overlayers of the reducing molecule decamethylcobaltocene (${\mathit{CoCp}}_2^1$) is demonstrated using photoelectron spectroscopy. A transfer doping model involving integer charge transfer between molecules reproduces quantitatively all measured level shifts as a function of ${\mathit{CoCp}}_2^1$ coverage using the ionisation potential of ${\mathit{CoCp}}_2^1$ and the electron affinity of ZnTPP as sole input parameters. The model yields the experimentally observed limitation of doping to the first monolayer of cobaltocene while further layers remain neutral without the need to resort to special bonding arrangements for the first monolayer. Temperature-dependent studies reveal that doping is still present at room temperature, despite the high vapour pressure of ${\mathit{CoCp}}_2^1$. Higher annealing temperatures initiate ${\mathit{CoCp}}_2^1$ molecular dissociation and diffusion of Co atoms into the ZnTPP film. Hence, the nature of doping changes from surface molecular transfer doping to bulk metallic doping as a function of temperature.     

Controlling the morphology of nanofiber-P3HT:PCBM blends for organic bulk heterojunction solar cells

Within the field of organic bulk heterojunction solar cells, the morphology of the active layer has a key role in obtaining high power conversion efficiencies. P3HT nanofibers, obtained in highly concentrated solutions, are able to give controlled morphologies directly upon deposition. Since the solar cell efficiency of fiber solar cells depends on the fiber content of the casting solution, it is important to control this parameter. Here, we demonstrate an easy way to control the fiber content in the casting solution, i.e. changing the solution temperature. By using solution heating, the overall molecular weight of the polymer in the blend is kept constant, fiber isolation is not needed and the use of solvent mixtures is avoided. The obtained optimal power conversion efficiency is shown to be linked to the morphology of the active layer, which is studied with Transmission Electron Microscopy (TEM).     

Modulation of work function of ITO by self-assembled monolayer and its effect on device characteristics of inverted perovskite solar cells

In this work, self-assembled monolayers of a series of para-substituted phenylphosphonic acids were formed on the ITO substrate surface for the fabrication of perovskite-based solar cells. With the perovskite layer directly transferred by a stamping method on the SAM-modified ITO surface, followed by spin-coated PCBM layer, an inverted-type, hole-transport layer-free solar cell was fabricated. The device characteristics, such as open circuit voltage V_oc, short circuit current J_sc, and field factor FF, were analyzed with respect to the energy level alignment at the ITO/perovskite interface. The best power conversion efficiency was observed with ITO having work function closest to the valence band maximum of perovskite, giving an efficiency of 13.94 %, considerably higher than the 8.64 % from the bare ITO-based device.     

The role of the density of interface states in interfacial energy level alignment of PTCDA

We introduce extensions to a recently developed numerical model to understand the origin of universal Fermi level pinning of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and its underlying mechanism. Our calculations are supported by ultraviolet and X-ray photoelectron spectroscopy to investigate the electronic structure of PTCDA on a wide range of substrates with different work functions and coupling interactions. For 20 nm thick layers of PTCDA, nearly unchanged hole injection barriers on all substrates are observed without any dependence on the type of substrate (unreactive, reactive or passivated metals and polymers). The simulation results demonstrate how the shape of the DOS near the interface has long-range influence on key parameters (e.g. the barrier to charge injection) of the entire organic film.     

Interfacial Energy Level Alignment and Defect Passivation by Using a Multifunctional Molecular for Efficient and Stable Perovskite Solar Cells

Tin oxide (SnO₂) is currently the dominating electron transport material (ETL) used in state-of-the-art perovskite solar cells (PSCs). However, there are amounts of defects distributed at the interface between ETL and perovskite to deteriorate PSC performance. Herein, a molecule bridging layer is built by incorporating 2,5-dichloroterephthalic acid (DCTPA) into the interface between the SnO₂ and perovskites to achieve better energy level alignment and superior interfacial contact. The multifunctional molecular bridging layer not only can passivate the trap states of Sn dangling bonds and oxygen vacancies resulting in improved conductivity and the electron extraction of SnO₂ but also can regulate the perovskite crystal growth and reduce defect-assisted nonradiative recombination due to its strong interaction with undercoordinated lead ions. As a result, the DCTPA-modified PSCs achieve champion power conversion efficiencies (PCEs) of 23.25% and 20.23% for an active area of 0.15 cm² device and 17.52 cm² mini-module, respectively. Moreover, the perovskite films and PSCs based on DCTPA modification show excellent long-term stability. The unencapsulated target device can maintain over 90% of the initial PCE after 1000 h under ambient air. This strategy guides design methods of molecule bridging layer at the interface between SnO₂ and perovskite to improve the performance of PSCs .     

The role of gap states on energy level alignment at an α-NPD/HAT(CN)6 charge generation interface

We report the effect of gap states on energy level alignment in a typical organic charge generation interface of N,N-bis(1-naphthyl)-N,N-diphenyl-1,1-biphenyl-4,4-diamine (α-NPD)/hexaazatriphenylene−hexacarbonitrile [HAT(CN)₆] by using ultraviolet and X-ray photoemission spectroscopy. The gap states tailed from the highest occupied molecular orbital (HOMO) onset of α-NPD dominate the Fermi level pinning at the α-NPD(<1.6 nm)/HAT(CN)₆ interface, which is favorable for charges generation upon bias operation and facilitates the electron injection from the HOMO-tail region of α-NPD to the lowest unoccupied molecular orbital (LUMO) region of HAT(CN)₆.     

Enhancing photovoltaic response of organic solar cells using a crystalline molecular template

We demonstrate that a crystalline pentacene molecular templating layer considerably changes the morphology of the subsequently deposited lead phthalocyanine (PbPc) layer, resulting in an improved crystallinity at the early stages of growth of the PbPc film and a higher content of the triclinic phase. For bilayer PbPc (20 nm)/C₆₀ (40 nm) organic solar cells with or without the pentacene templating layer, the use of the pentacene templating layer leads to a 48% enhancement in the short-circuit current without noticeably affecting the solar cell open-circuit voltage or fill factor. A copper or zinc phthalocyanine molecular templating layer also leads to enhanced photovoltaic response from the PbPc/C₆₀ cells, though less significant than the pentacene template. The improved device performance originates from stronger absorption by the triclinic PbPc phase in the near infrared and the enhanced internal quantum efficiency over the entire spectrum where PbPc absorbs.     

Manipulation of the electronic and photovoltaic properties of materials based on small push-pull molecules by substitution of the arylamine donor block by aliphatic groups

Push-pull molecules with an arylamine donor block connected to a dicyanovinyl acceptor via a thienyl π-conjugating spacer have been synthesized in order to analyze the effects of replacing an outer phenyl ring of the triphenylamine (TPA) block of a reference compound by methyl, hexyl, heptafluoropentyl and dioxaoctyl groups. Optical, electrochemical and X-ray diffraction data show that these substitutions have minor influence on the energy levels of the molecule but exert a considerable effect on the structure, optical, electrical and photovoltaic properties of the resulting materials.     

Efficient perovskite solar cells using trichlorosilanes as perovskite/PCBM interface modifiers

Interfaces are crucial for high-performance perovskite solar cells. Here, phenyltrichlorosilane (PTS) and octadecyltrichlorosilane (OTS) were used to modify the interface between perovskite layer and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) layer in an inverted layered perovskite device. Such treatments facilitated the formation of a high-quality PCBM film and effectively decreased the density of surface traps that induce undesirable electron-hole recombination. As a result, the average power conversion efficiency of PTS (and OTS) modified devices was improved from 9.60% to 11.96% (and 11.08%), with a highest value of 12.63% (and 11.87%). Therefore, this study provides an attractive mothed to improve the quality of PCBM film on top of perovskite layer and finally the performance of inverted perovskite solar cells.     

Novel cathode buffer layer of Ag-doped bathocuproine for small molecule organic solar cell with inverted structure

2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (C₂₆H₂₀N₂), known as bathocuproine (BCP), is a commonly used cathode buffer layer in conventional structure organic solar cells (OSCs). We demonstrated that BCP layer can also be used as a buffer layer in inverted structure OSCs. Unfortunately, the device exhibited an anomalous kink in the current density–voltage (J–V) characteristics, namely, an S-shaped J–V curve, leading to a low fill factor and low power conversion efficiency (PCE). To improve device performance, Ag-doped BCP layer (Ag:BCP) was used to replace the BCP layer. The results showed that the Ag:BCP layer can eliminate the S-kink in the J–V curve, resulting in a large improvement of fill factor and PCE. The origin of the S-shaped J–V curve was demonstrated to originate from the charge accumulation at the fullerene (C₆₀)/BCP interface. On the contrary, the C₆₀/Ag:BCP interface has favorable electronic properties with beneficial gap states for the transport of free carriers. Together with the good conductivity of Ag:BCP layer and the smooth morphology properties, the device performance was greatly improved by Ag:BCP buffer layer.     

Improved performance of organic photovoltaic devices by doping F4TCNQ onto solution-processed graphene as a hole transport layer

Interfacial engineering is crucial for the stability and efficiency of organic solar cells. PEDOT:PSS, which has been widely used as a hole transport layer, has stability issues when exposed to air because of its acidic and hygroscopic nature. Herein, we investigated the electrical properties of reduced graphene oxide covered with an F₄TCNQ interfacial layer as an alternative and its effect on the photovoltaic performance. Using an array of charge transport, spectroscopic and imaging techniques we found that the reduced graphene oxide film is efficiently hole-doped through an interfacial charge transfer, which enhances its electrical properties and favorably modifies its work function. Consequently, the open-circuit voltage and fill factor of solar cells incorporating such films are improved. P3HT might also be hole-doped by F₄TCNQ, due to the formation of an intermixed interfacial layer, resulting in an increase of power conversion efficiency.     

High performance organic solar cells enabled by an iodinated additive

Additive engineering is a simple and effective strategy to enhance the efficiency of organic solar cells (OSCs). However, traditional additives such as 1,8-diiodooctane (DIO) or 1-chloronaphthalene (CN), suffer from inferior stability, concentration sensitivity, and need additional thermal treatments, which are not desirable for industrial application. Here we introduce a simple, effective and versatile solid additive 1,3-diiodobenzene (1,3-DIB) into the OSCs. In comparison to the control devices, the 1,3-DIB treated OSCs exhibit significantly improved performance with a power conversion efficiency (PCE) of 16.90% for polymer OSCs and a PCE of 14.35% for binary all-small-molecule OSCs. Mechanism studies reveal that 1,3-DIB can improve charge transport and extraction, decrease charge recombination, enhance crystallinity and improve the phase separation. Furthermore, no thermal annealing is needed in PM6:Y6 based OSCs and the 1,3-DIB treated devices show excellent stability and reproducibility in both polymer and small molecule OSCs. Our results demonstrated that additive engineering is a powerful method to enhance the OSC performance.     

Porphyrin small molecules containing furan- and selenophene-substituted diketopyrrolopyrrole for bulk heterojunction organic solar cells

Two porphyrin small molecules substituted with either furan- or selenophene-linked diketopyrrolopyrrole units are designed and synthesized. The impacts of the O and Se chalcogen atoms in the linking 5-membered ring in these new donor materials on the performance of organic solar cells are discussed and contrasted with the previously described thiophene analogue. The heavy atoms broaden the absorption and narrow the bandgap by tuning the energy levels regularly. Furthermore, the selenophene containing analogue shows better miscibility and smaller phase separation with [6,6]-phenyl C₆₁-butyric acid methyl ester (PC61BM) than the furan analogue. The optimized organic solar cells based on the furan and the selenophene containing species are achieved in the presence of pyridine and 1,8-diiodooctane additives with power conversion efficiencies of 4.3% and 5.8%, respectively, under simulated AM 1.5 illumination (100 mW cm⁻²).