Recent Advances in Bulk Heterojunction Polymer Solar Cells

Technological deployment of organic photovoltaic modules requires improvements in device light-conversion efficiency and stability while keeping material costs low. Here we demonstrate highly efficient and stable solar cells using a ternary approach, wherein two non-fullerene acceptors are combined with both a scalable and affordable donor polymer, poly(3-hexylthiophene) (P3HT), and a high-efficiency, low-bandgap polymer in a single-layer bulk-heterojunction device. The addition of a strongly absorbing small molecule acceptor into a P3HT-based non-fullerene blend increases the device efficiency up to 7.7 ± 0.1% without any solvent additives. The improvement is assigned to changes in microstructure that reduce charge recombination and increase the photovoltage, and to improved light harvesting across the visible region. The stability of P3HT-based devices in ambient conditions is also significantly improved relative to polymer:fullerene devices. Combined with a low-bandgap donor polymer (PBDTTT-EFT, also known as PCE10), the two mixed acceptors also lead to solar cells with 11.0 ± 0.4% efficiency and a high open-circuit voltage of 1.03 ± 0.01 V. Ternary organic blends using two non-fullerene acceptors are shown to improve the efficiency and stability of low-cost solar cells based on P3HT and of high-performance photovoltaic devices based on low-bandgap donor polymers.

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Reducing the efficiency–stability–cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells

Organic solar cells (OSCs) present some advantages, such as simple preparation, light weight, low cost and large-area flexible fabrication, and have attracted much attention in recent years. Although the power conversion efficiencies have exceeded 10%, the inferior device stability still remains a great challenge. In this review, we summarize the factors limiting the stability of OSCs, such as metastable morphology, diffusion of electrodes and buffer layers, oxygen and water, irradiation, heating and mechanical stress, and survey recent progress in strategies to increase the stability of OSCs, such as material design, device engineering of active layers, employing inverted geometry, optimizing buffer layers, using stable electrodes and encapsulation. Some research areas of device stability that may deserve further attention are also discussed to help readers understand the challenges and opportunities in achieving high efficiency and high stability of OSCs towards future industrial manufacture.

Stability of organic solar cells: challenges and strategies

Improving the long-term stability of perovskite solar cells is critical to the deployment of this technology. Despite the great emphasis laid on stability-related investigations, publications lack consistency in experimental procedures and parameters reported. It is therefore challenging to reproduce and compare results and thereby develop a deep understanding of degradation mechanisms. Here, we report a consensus between researchers in the field on procedures for testing perovskite solar cell stability, which are based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols. We propose additional procedures to account for properties specific to PSCs such as ion redistribution under electric fields, reversible degradation and to distinguish ambient-induced degradation from other stress factors. These protocols are not intended as a replacement of the existing qualification standards, but rather they aim to unify the stability assessment and to understand failure modes. Finally, we identify key procedural information which we suggest reporting in publications to improve reproducibility and enable large data set analysis. Reliability of stability data for perovskite solar cells is undermined by a lack of consistency in the test conditions and reporting. This Consensus Statement outlines practices for testing and reporting stability tailoring ISOS protocols for perovskite devices.

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Consensus statement for stability assessment and reporting for perovskite photovoltaics based on ISOS procedures

This Review highlights the large number of methods to exploit colloidal assembly of comparably simple particles with nano- to micrometer dimensions in order to access complex structural hierarchies from nanoscopic over microscopic to macroscopic dimensions

Advances in colloidal assembly: the design of structure and hierarchy in two and three dimensions.

A stable gel of Au nanoparticles in polydimethylsiloxane (PDMS) nanocomposite is prepared by employing the curing agent of PDMS elastomer as a reducing agent for the formation of Au nanoparticles by an in-situ process. The viscoelastic nature of these gels is very sensitive to the Au nanoparticle loading and the synthetic temperature conditions. Even a very low Au content of 0.09 wt% is sufficient enough to bring in the transition from sponge state to gel state at room temperature. Higher synthetic temperature also forms sponge formation. Infrared and ultraviolet–visible spectroscopy measurements have provided insight into PDMS crosslinking and nanoparticle formation, respectively. The optimization of the gel properties can have direct influence on the processability of Au nanoparticle–PDMS nanocomposite gels, with interesting implications in electronic, optical and microfluidic devices.

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Viscoelastic nature of Au nanoparticle–PDMS nanocomposite gels

Layer-by-Layer Coating of Cobalt-Based Ink for Large-Scale Fabrication of OER Electrocatalyst

A stretchable and compliant electrode surface of Au metal on polydimethylsiloxane (PDMS) is introduced in this study. A thin layer of Au nanoparticles is thus formed by a simple chemical reduction from aqueous Au salt solution with the AuPDMS gel surface itself acting as a reducing site. Employing the swelling behaviour of PDMS and Au nanoparticles affinity to bind with sulphur, an in-plane molecular device has been realized for measuring the conductance of thiol molecules. The device is capable of forming stable and robust linkages with Au. The molecules anchored between the Au islands are able to undergo reversible compression and tension, which shows the flexibility of the device.

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Fabrication of stretchable compliant electrodes on PDMS with Au nanoparticles

As solar cell modules are becoming larger, it is important to pay attention to defects originating from the fabrication process and degradation during operation in the ambient. In this article, a simple method of using computer screen display as a light source to map the photoresponse of the solar cells, is reported. The method requires only a conventional computer loaded with a software code that enables a light spot of defined size to raster scan across the cell area as the photogenerated voltage is read out by a voltmeter using a USB connection. Screen-display-induced photoresponse (SDIP) mapping is an enabling technique to reveal the defective regions in the active layer as well as at the electrode interface, which, in many instances, cannot be deciphered simply by visual examination. Spectral response mapping by using light spots of different colors is also possible.

Screen-Display-Induced Photoresponse Mapping for Large-Area Photovoltaics

A simple, one-step direct micromolding process has been realised to produce highly conducting Pd micro-stripes over large areas on various substrates including flexible polyimide. Under a PDMS micromold, Pd octanethiolate served as a precursor at 250 degrees C, a temperature at which the precursor gets neatly metallised. Thus produced micro-stripes are robust under bending and can be utilised for flexible electronics. Hydrogen sensing by Pd micro-stripes is demonstrated. By electrolessly depositing Cu on the stripes, they can be made to peel off to form free standing Cu-Pd micro-ribbons.

Ritu Gupta

Papers

Viscoelastic nature of Au nanoparticle–PDMS nanocomposite gels

Layer-by-Layer Coating of Cobalt-Based Ink for Large-Scale Fabrication of OER Electrocatalyst

Fabrication of stretchable compliant electrodes on PDMS with Au nanoparticles

Screen-Display-Induced Photoresponse Mapping for Large-Area Photovoltaics

Direct micromolding of Pd micro-stripes for electronic applications.