Liuyanfeng

A novel, yet simple solution fabrication technique to address the trade-off between photocurrent and fill factor in thick bulk heterojunction organic solar cells is described. The inverted off-center spinning technique promotes a vertical gradient of the donor–acceptor phase-separated morphology, enabling devices with near 100% internal quantum efficiency and a high power conversion efficiency of 10.95%.

Rigid fused perylene diimide (PDI) dimers bridged with heterocycles exhibit superior photovoltaic performance compared to their unfused semiflexible analogues. Changing the chalcogen atoms in the aromatic bridges gradually increases the twist angles between the two PDI planes, leading to a varied morphology in which the one bridged by thiophene achieves a balance and shows the best efficiency of 6.72%.

It is commonly believed that the work-function reduction effect of the cathode interfacial material in organic electronic devices leads to better energy-level alignment at the organic/electrode interface, which enhances the device performance. However, there is no agreement on the exact dipole direction in the literature. In this study, a peel-off method to reveal the buried organic/metal interface to examine the energy-level alignment is developed. By splitting the device at different interfaces, it is discovered that oppositely oriented dipoles are formed at different surfaces of the interfacial layer. Moreover, the function of the electrode interface differs in different device types. In organic light-emitting diodes, the vacuum-level alignment generally occurs at the organic/cathode interface, while in organic photovoltaic devices, the Fermi-level pinning commonly happens. Both are determined by the integer charge-transfer levels of the organic materials and the work-function of the electrode. As a result, the performance enhancement by the cathode interfacial material in organic photovoltaic devices cannot be solely explained by the energy-level alignment. The clarification of the energy-level alignment not only helps understand the device operation but also sets up a guideline to design the devices with better performance.

There is no agreement on the dipole direction of the cathode interfacial layer in organic electronic devices in the literature. By splitting the device at different interfaces to study the energy-level alignment, the energetic diagrams of the organic light-emitting diode (OLED) and the organic photovoltaic (OPV) device are clarified. The vacuum level is aligned across the organic/metal interface in OLED, while energy pinning occurs in OPV.

For polymer solar cells (PSCs) with conventional configuration, the vertical composition profile of donor:acceptor in active layer is detrimental for charge carrier transporting/collection and leads to decreased device performance. A cross-linkable donor polymer as the underlying morphology-inducing layer (MIL) to tune the vertical composition distribution of donor:acceptor in the active layer for improved PSC device performance is reported. With poly(thieno[3,4-b]-thiophene/benzodithiophene):[6,6]-phenyl C₇₁-butyric acid methyl ester (PTB7:PC₇₁BM) as the active layer, the MIL material, PTB7-TV, is developed by attaching cross-linkable vinyl groups to the side chain of PTB7. PSC device with PTB7-TV layer exhibits a power conversion efficiency (PCE) of 8.55% and short-circuit current density (J_SC) of 15.75 mA cm⁻², in comparison to PCE of 7.41% and J_SC of 13.73 mA cm⁻² of the controlled device. The enhanced device performance is ascribed to the much improved vertical composition profile and reduced phase separation domain size in the active layer. These results demonstrate that cross-linked MIL is an effective strategy to improve photovoltaic performance of conventional PSC devices.

A cross-linkable donor polymer is developed and used as the underlying layer to improve the vertical composition distribution of donor:acceptor in the active layer of polymer solar cells (PSCs). With the improvement, the regular PSC device based on PTB7:PC₇₁BM active layer exhibits power conversion efficiency increase from 7.41% to 8.55%.

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is less selective for holes in an inverted-architecture organic photovoltaic (OPV) than it is in a conventional-architecture OPV device due to differences between the interfacial-PSS concentration at the top and bottom of the PEDOT:PSS layer. In this work, thin layers of polysulfonic acids are inserted between the poly(3-hexylthiophene) (P3HT):indene-C₆₀ bisadduct (ICBA) bulk heterojunction (BHJ) active layer and PEDOT:PSS to create a higher concentration of acid at this interface and, therefore, mimic the distribution of materials present in a conventional device. Upon thermal annealing, this acid layer oxidizes P3HT, creating a thin p-type interlayer of P3HT⁺/acid⁻ on top of the BHJ. Using X-ray absorption spectroscopy, Kelvin probe, and ellipsometry measurements, this P3HT⁺/acid⁻ layer is shown to be insoluble in water, indicating it remains intact during the subsequent deposition of PEDOT:PSS. Current density–voltage measurements show this doped interlayer reduces injected dark current while increasing both open-circuit voltage and fill factor through the creation of a more hole selective BHJ-PEDOT:PSS interface.

Thin layers of sulfonic acid polymers are used to create a doped interlayer in organic photovoltaic devices. The presence of a p-type doped interlayer between the bulk heterojunction (BHJ) and the PEDOT:PSS hole transport layer (HTL) increases the hole selectivity of the BHJ-HTL interface, which increases the open-circuit voltage and fill factor.

Fast camera-based luminescence-imaging measurements on perovskite solar cells are presented. The fundamental correlation between the luminescence intensity and the open circuit voltage predicted by the generalised Planck law is confirmed, enabling various quantitative methods for the detection of efficiency-limiting defects to be applied to this new cell structure. Interstinegly, it is found that this fundamental correlation is valid only for light-soaked devices. Copyright © 2015 John Wiley & Sons, Ltd.

Fast (0.8 s) camera-based (a) photoluminescence and (b) electroluminescence-imaging measurements on perovskite solar cells.

In this work, alcohol-vapor solvent annealing treatment on CH₃NH₃PbI₃ thin films is reported, aiming to improve the crystal growth and increase the grain size of the CH₃NH₃PbI₃ crystal, thus boosting the performance of perovskite photovoltaics. By selectively controlling the CH₃NH₃I precursor, larger-grain size, higher crystallinity, and pinhole-free CH₃NH₃PbI₃ thin films are realized, which result in enhanced charge carrier diffusion length, decreased charge carrier recombination, and suppressed dark currents. As a result, over 43% enhanced efficiency along with high reproducibility and eliminated photocurrent hysteresis behavior are observed from perovskite hybrid solar cells (pero-HSCs) where the CH₃NH₃PbI₃ thin films are treated by methanol vapor as compared with that of pristine pero-HSCs where the CH₃NH₃PbI₃ thin films are without any alcohol vapor treatment. In addition, the dramatically restrained dark currents and raised photocurrents give rise to over ten times enhanced detectivities for perovskite hybrid photodetectors, reaching over 10¹³ cm Hz^1/2 W⁻¹ (Jones) from 375 to 800 nm. These results demonstrate that the method provides a simple and facile way to boost the device performance of perovskite photovoltaics.

High performance of perovskite photovoltaics (perovskite solar cells and perovskite photodetectors) is realized by alcohol-vapor solvent annealing treatment on CH₃NH₃PbI₃ thin films to enhance the crystal growth and the grain size of the CH₃NH₃PbI₃ crystals.

A universal strategy for efficient light trapping through the incorporation of gold nanorods on the electron transport layer (rear) of organic photovoltaic devices is demonstrated. Utilizing the photons that are transmitted through the active layer of a bulk heterojunction photovoltaic device and would otherwise be lost, a significant enhancement in power conversion efficiency (PCE) of poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]:phenyl-C₇₁-butyric acid methyl ester (PCDTBT:PC₇₁BM) and 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):PC₇₁BM by ≈13% and ≈8%, respectively. PCEs over 8% are reported for devices based on the PTB7:PC₇₁BM blend. A comprehensive optical and electrical characterization of our devices to clarify the influence of gold nanorods on exciton generation, dissociation, charge recombination, and transport inside the thin film devices is performed. By correlating the experimental data with detailed numerical simulations, the near-field and far-field scattering effects are separated of gold nanorods (Au NRs), and confidently attribute part of the performance enhancement to the enhanced absorption caused by backscattering. While, a secondary contribution from the Au NRs that partially protrude inside the active layer and exhibit strong near-fields due to localized surface plasmon resonance effects is also observed but is minor in magnitude. Furthermore, another important contribution to the enhanced performance is electrical in nature and comes from the increased charge collection probability.

High efficiency organic photovoltaic (OPV) devices are fabricated based a novel and universal light trapping mechanism, using gold nanorods (Au NRs) as back contact reflectors. The incorporation of Au NRs inside the back contact interfacial layer (titanium sub oxide) gives rise to a device efficiency of ≈13%, compared to the record performance of 8.25%. This is revealed to be mainly due to scattering by a combination of theoretical and experimental results.

The results of a meta-analysis conducted on organic photovoltaics (OPV) lifetime data reported in the literature is presented through the compilation of an extensive OPV lifetime database based on a large number of articles, followed by analysis of the large body of data. We fully reveal the progress of reported OPV lifetimes. Furthermore, a generic lifetime marker has been defined, which helps with gauging and comparing the performance of different architectures and materials from the perspective of device stability. Based on the analysis, conclusions are drawn on the bottlenecks for stability of device configurations and packaging techniques, as well as the current level of OPV lifetimes reported under different aging conditions. The work is summarized by discussing the development of a tool for OPV lifetime prediction and the development of more stable technologies. An online platform is introduced that will aid the process of generating statistical data on OPV lifetimes and further refinement of the lifetime prediction tool.

The results of a meta-analysis of organic photovoltaics (OPVs) lifetimes are presented. These results illuminate some major bottlenecks, and reveal the current status and progress of the stability of OPVs. A generic marker is additionally introduced that allows the comparison of the lifetimes of various samples tested under different conditions.

Transparent electrodes (TEs) having electrooptical trade-offs better than state-of-the-art indium tin oxide (ITO) are continuously sought as they are essential to enable flexible electronic and optoelectronic devices. In this work, a TiO₂-Ag-ITO (TAI)-based TE is introduced and its use is demonstrated in an inverted polymer solar cell (I-PSCs). Thanks to the favorable nucleation and wetting conditions provided by the TiO₂, the ultrathin silver film percolates and becomes continuous with high smoothness at very low thicknesses (3–4 nm), much lower than those required when it is directly deposited on a plastic or glass substrate. Compared to conventional ITO-TE, the proposed TAI-TE exhibits exceptionally lower electrical sheet resistance (6.2 Ω sq⁻¹), higher optical transmittance, a figure-of-merit two times larger, and mechanical flexibility, the latter confirmed by the fact that the resistance increases only 6.6% after 10³ tensile bending cycles. The I-PSCs incorporating the TAI-TE show record power conversion efficiency (8.34%), maintained at 96% even after 400 bending cycles.

TiO₂/Ag/indium tin oxide (ITO)-based transparent electrodes (TEs) with sheet resistance of 6.2 Ω sq⁻¹ and average optical transmittance in the visible of 87.6% are developed. These performances are superior to those of state-of-the-art single-layer ITO. Polymer solar cells employing such TEs achieve 8.34% efficiency, higher than similar structures on conventional ITO.

A life cycle assessment case study involving organic photovoltaic technology using phenyl-C₆₁-butyric acid methyl ester and poly(3-hexylthiophene) is presented. Although solar technology converts freely available solar radiation into more useful forms of energy, potential environmental impacts can occur during the life cycle of the product. A cradle-to-gate life cycle assessment is completed, comparing organic solar cells with traditional silicon-based cells across 18 multiple criteria. The functional unit is defined as the production of 1 watt-peak of electricity produced. The inventory is based on prospective organic solar cell technology and two traditional silicon technologies. The results demonstrate that from a life cycle perspective, organic solar cells can outperform conventional silicon solar cells with impacts reduced by 93%. The energy payback time for the default organic photovoltaic cell was 0.21 years (75 days) compared with multicrystalline silicon and amorphous silicon's 2.7 and 2.2 years, respectively. The minimum required lifetime of the organic cells, so that their impacts were no worse than amorphous silicon's over 25 years, was measured between 1.2 and 8.9 years. Results of the sensitivity analysis demonstrate that consideration of manufacturing routes (e.g., fullerene or solar cell production) can be targeted using life cycle assessment for further improvements in the environmental, human health, and ecotoxicity profile of organic solar cells. Copyright © 2015 John Wiley & Sons, Ltd.

From a life cycle perspective, organic photovoltaic cell performance is promising. Shown here is the minimum required lifetime of organic cells needed for environmental impact parity compared with thin-film silicon cells having a 25-year lifetime.

A new series of organic salts with selective near-infrared (NIR) harvesting to 950 nm is reported, and anion selection and blending is demonstrated to allow for fine tuning of the open-circuit voltage. Extending photoresponse deeper into the NIR is a significant challenge facing small molecule organic photovoltaics, and recent demonstrations have been limited by open-circuit voltages much lower than the theoretical and practical limits. This work presents molecular design strategies that enable facile tuning of energy level alignment and open-circuit voltages in organic salt-based photovoltaics. Anions are also shown to have a strong influence on exciton diffusion length. These insights provide a clear route toward achieving high efficiency transparent and panchromatic photovoltaics, and open up design opportunities to rapidly tailor molecules for new donor–acceptor systems.

Organic salt photovoltaics with open-circuit voltages approaching the excitonic limit are demonstrated. Anion exchange is shown to enhance energy level alignment and nearly double the exciton diffusion length. Anion alloying enables a design strategy for fine energy level tuning to simultaneously optimize open-circuit voltage and photocurrent in new organic salt systems for multijunction and transparent phovoltaics with deeper near-infrared response.

In article number 1500761, Jongseung Yoon and co-workers report a photovoltaic system that can improve the collection efficiency of above-bandgap, long-wavelength photons in ultrathin nanostructured silicon solar cells. A flexible composite solar module integrated with silver nanostructures and upconversion printing medium enables the enhanced photovoltaic performance of surface-embedded 8-μm-thick silicon microcells via synergistic contributions from plasmonically amplified spectral upconversion, waveguiding, and fluorescence of the upconversion medium.

A variety of metal fluorides, including lithium fluoride (LiF), magnesium fluoride (MgF₂), barium fluoride (BaF₂), strontium fluoride (SrF₂), aluminum fluoride (AlF₃), zirconium fluoride (ZrF₄), and cerium fluoride (CeF₃), are used as the cathode interfacial layer (CIL) in polymer photovoltaic cells to assess their effect on device performance. CeF₃, BaF₂, and SrF₂ CILs exhibit better performance than a typical LiF CIL. The SrF₂-based device shows a power conversion efficiency (PCE) of 7.17%, which is approximately 9% higher than that of the LiF-based device; this, to our knowledge, is the first report on the SrF₂-based organic photovoltaic cell device. The open-circuit voltage (V _OC) and fill factor (FF) of the fluoride-based devices are correlated to the work functions (WFs) of the corresponding metals, which in turn influence the PCE. X-ray photoelectron spectroscopy measurements of fluoride-based cathodes reveal the occurrence of a displacement reaction and an interfacial dipole at the fluoride/aluminum interface, which lead to a reduced effective WF of the cathode and improved charge collection. Consequently, an improved PCE is achieved together with an increased V _OC and FF.

Organic photovoltaic cells with different metal fluorides as the cathode interfacial layers are fabricated. Parameters of the fluoride-based devices depend on the work function of the corresponding pure metal in the fluoride as a result of the displacement reaction and dipole at the metal fluoride/Al interface.

Partially converted poly(1,4-phenylenevinylene) (PPV) is used as a thin p-type interlayer in planar organohalide pervoskite solar cells resulting in a high V_oc, 1.06 V, a power conversion efficiency of ≈15%, and minimized charge carrier recombination.

Photo-conductive AFM spectroscopy (‘pcAFMs’) is proposed as a high-resolution approach for investigating nanostructured photovoltaics, uniquely providing nanoscale maps of photovoltaic (PV) performance parameters such as the short circuit current, open circuit voltage, maximum power, or fill factor. The method is demonstrated with a stack of 21 images acquired during in situ illumination of micropatterned polycrystalline CdTe/CdS, providing more than 42 000 I/V curves spatially separated by ~5 nm. For these CdTe/CdS microcells, the calculated photoconduction ranges from 0 to 700 picoSiemens (pS) upon illumination with ~1.6 suns, depending on location and biasing conditions. Mean short circuit currents of 2 pA, maximum powers of 0.5 pW, and fill factors of 30% are determined. The mean voltage at which the detected photocurrent is zero is determined to be 0.7 V. Significantly, enhancements and reductions in these more commonly macroscopic PV performance metrics are observed to correlate with certain grains and grain boundaries, and are confirmed to be independent of topography. These results demonstrate the benefits of nanoscale resolved PV functional measurements, reiterate the importance of microstructural control down to the nanoscale for 'PV devices, and provide a widely applicable new approach for directly investigating PV materials. Copyright © 2015 John Wiley & Sons, Ltd.

Photo-conductive AFM spectroscopy (‘pcAFMs’) is proposed as a high-resolution approach for investigating photovoltaics, uniquely providing nanoscale maps of photovoltaic (PV) performance metrics. The method is demonstrated on micropatterned polycrystalline CdTe/CdS solar cells. The calculated photoconduction ranges from 0 to 700 picoSiemens (pS) upon ~1.6 suns of in situ illumination, depending on location and biasing conditions. Mean short circuit currents of 2 pA, open circuit voltages of 0.8 V, maximum powers of 0.5 pW, and fill factors of 30% are determined.