wangzhaowei

It is reported herein that electrical performance of diketopyrrolopyrrole based small-molecule solar cells are boosted via increasing the conjugation size fraction (δ) on lateral side-chains and on the mainchain of small-molecule donor. The conjugation size fraction is defined as δ = N _p/N _m, where N _p is the number of five- or six-member aromatic rings on the peripheral side-chains and N _m is that on the mainchain. As the DPP-dimer's core structure varies from 5,10-bis((5-(2-ethylhexyl)thiophen-2-yl)dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DT-BDT) to 4,8-bis((5-(2-ethylhexyl)thiophen-2-yl)-BDT (2T-BDT), and then 4,8-bis(5′-(2-ethylhexyl)-(2,2′-bithiophen)-5-yl)-BDT (4T-BDT), the term of δ increases from 2/13 to 2/11 and then 4/11. In the blended film with the fullerene acceptor, the donor phase size decreases from 35 to 24 and 15 nm, becoming more and more approaching to the effective exciton diffusion length. Consequently, the obtainable maximum generation rate of electron–hole bound pairs increases from 7.0 × 10²⁷ to 7.3 × 10²⁷ and then 7.7 × 10²⁷ m⁻³ s⁻¹. Meanwhile, reduction of donor crystallinity with the increase of δ leads to a higher effective carrier mobility, which suppresses recombination losses and leads to a larger carrier collection probability. Promotions of both charge separation and transport give a larger short-circuit current density, a higher fill-factor, and ultimately a larger efficiency.

Relative size of conjugation systems on lateral side-chains relative to that on the mainchain of a small molecule donor is found to be a structural factor that effectively tunes donor phase size and molecular crystallinity, and consequently on small-molecule organic solar cell performance.

On-fabrication solid-state N-doping of graphene using a fluorosurfactant (Zonyl)-added ZnO layer on a graphene surface is developed by Tae-Woo Lee and co-workers, in article number 1600172. The N-doping facilitates efficient electron transfer from the ZnO layer to the graphene cathode. Based on this, inverted organic solar cells with a power conversion efficiency of 7.5% are fabricated, which is 100% power conversion efficiency with respect to the ITO cathode.

Organometal-trihalide-perovskite-based solar cells have exhibited high efficiencies when incorporated into mesoscopic NiO (NiO_nc) hole-transport layers. The integration of a NiO_nc-perovskite heterojunction provides an inorganic alternative as a p-type contact material with efficient hole extraction for perovskite-based solar cells. Herein the origin of such highly efficient carrier transport is studied in terms of electronic, chemical and transport properties of a NiO_nc-perovskite heterojunction with X-ray photoelectron spectra, ultraviolet photoelectron spectra, near-edge X-ray absorption fine structure spectra, a scanning transmission X-ray microscope, and calculations of electronic structure. A pronounced chemical redox reaction is found at an NiO_nc-perovskite heterojunction such that PbI₂ is oxidized to PbO with subsequent formation of hole-dopant CH₃NH₃PbI_3–2δO_δ at the heterojunction. The generation of hole-doping CH₃NH₃PbI_3–2δO_δ induced by the redox reaction at the NiO_nc/perovskite heterojunction plays a significant role to facilitate the carrier transport, and thus enhances the photovoltaic efficiencies.

A pronounced chemical redox reaction at an NiO_nc-perovskite heterojunction to form the hole-dopant CH₃NH₃PbI_3–2δO_δ is determined. This interfacial hole-doping CH₃NH₃PbI_3–2δO_δ and NiO_nc exhibits superior electronic properties to bring about the energy matching of NiO_nc/perovskite interface and enable faster hole transport.

Organic–inorganic halide perovskite has received extensive attention as a light harvester for next-generation low-cost and high-performance photovoltaics. Its superior optoelectronic properties are attractive among most thin film absorber materials, such as an extremely high absorption coefficient, optimal band gap, ambipolar carrier transport property, and high defects tolerance. However, it requires suitable electrodes and carrier transport materials to fulfill efficient photovoltaic process within an entire device. Thus, the interfaces along the device play a crucial role in determining device photovoltaic performance. Here, the progress of understanding interfaces in perovskite photovoltaics is reviewed from the perspective of processing chemistry and photophysics of carriers, which are the key parameters for the corresponding device photovoltaic behavior. This study is mainly focused on the relevant working mechanism, interface design fundamentals, and the resulting carrier dynamic control over the entire architecture. The study of the interfaces with appropriate materials design provides a fundamental understanding of the photocarrier behavior, including separation, transportation, and collection. The accumulative efforts will contribute to the construction of high-efficiency perovskite-based single junction and multijunction photovoltaic devices. It also affects other properties of perovskite solar cells, including J–V hysteresis phenomenon, and long-term stability. Suggestions with respect to required improvements and future research directions are provided based on the current field of available literature.

The progress of interfaces design in perovskite photovoltaics is reviewed, from the perspective of various aspects, including the relevant working mechanism, interface design fundamentals, devices long-term stability, as well as hysteresis phenomenon, etc., which are the key parameters for the corresponding materials understanding and device photovoltaic behavior.

Perovskite solar cells based on CH₃NH₃PbBr₃ with a band gap of 2.3 eV are attracting intense research interests due to their high open-circuit voltage (V_oc) potential, which is specifically relevant for the use in tandem configuration or spectral splitting. Many efforts have been performed to optimize the V_oc of CH₃NH₃PbBr₃ solar cells; however, the limiting V_oc (namely, radiative V_oc:V_oc,rad) and the corresponding ΔV_oc (the difference between V_oc,rad and V_oc) mechanism are still unknown. Here, the average V_oc of 1.50 V with the maximum value of 1.53 V at room temperature is achieved for a CH₃NH₃PbBr₃ solar cell. External quantum efficiency measurements with electroluminescence spectroscopy determine the V_oc,rad of CH₃NH₃PbBr₃ cells with 1.95 V and a ΔV_oc of 0.45 V at 295 K. When the temperature declines from 295 to 200 K, the obtained V_oc remains comparably stable in the vicinity of 1.5 V while the corresponding ΔV_oc values show a more significant increase. Our findings suggest that the V_oc of CH₃NH₃PbBr₃ cells is primarily limited by the interface losses induced by the charge extraction layer rather than by bulk dominated recombination losses. These findings are important for developing strategies how to further enhance the V_oc of CH₃NH₃PbBr₃-based solar cells.

CH₃NH₃PbBr₃ solar cells with the average open circuit voltage of 1.50 V are achieved. External quantum efficiency measurements and electroluminescence spectroscopy are employed to predict the limiting open circuit voltage and the corresponding voltage loss mechanism are clarified via temperature dependent measurements, beneficial for further open circuit voltage improvements of CH₃NH₃PbBr₃ solar cells.

Perovskite photovoltaics (PVs) have attracted attention because of their excellent power conversion efficiency (PCE). Critical issues related to large-area PV performance, reliability, and lifetime need to be addressed. Here, it is shown that doped metal oxides can provide ideal electron selectivity, improved reliability, and stability for perovskite PVs. This study reports p-i-n perovskite PVs with device areas ranging from 0.09 cm² to 0.5 cm² incorporating a thick aluminum-doped zinc oxide (AZO) electron selective contact with hysteresis-free PCE of over 13% and high fill factor values in the range of 80%. AZO provides suitable energy levels for carrier selectivity, neutralizes the presence of pinholes, and provides intimate interfaces. Devices using AZO exhibit an average PCE increase of over 20% compared with the devices without AZO and maintain the high PCE for the larger area devices reported. Furthermore, the device stability of p-i-n perovskite solar cells under the ISOS-D-1 is enhanced when AZO is used, and maintains 100% of the initial PCE for over 1000 h of exposure when AZO/Au is used as the top electrode. The results indicate the importance of doped metal oxides as carrier selective contacts to achieve reliable and high-performance long-lived large-area perovskite solar cells.

Doped metal oxides provide ideal electron selectivity, improved lifetime, and reliability for large-area perovskite-based photovoltaics. The proposed aluminum-doped zinc oxide electron selective contact provides suitable energy levels for carrier selectivity and stability, neutralizes the presence of pinholes, provides intimate interfaces, and maintains high power conversion efficiency for large-area solar cell devices.

Over the past five years, a rapid progress in organometal-halide perovskite solar cells has greatly influenced emerging solar energy science and technology. In perovksite solar cells, the overlying hole transporting material (HTM) is critical for achieving high power conversion efficiencies (PCEs) and for protecting the air-sensitive perovskite active layer. This study reports the synthesis and implementation of a new polymeric HTM series based on semiconducting 4,8-dithien-2-yl-benzo[1,2-d;4,5-d′]bistriazole-alt-benzo[1,2-b:4,5-b′]dithiophenes (pBBTa-BDTs), yielding high PCEs and environmentally-stable perovskite cells. These intrinsic (dopant-free) HTMs achieve a stabilized PCE of 12.3% in simple planar heterojunction cells—the highest value to date for a polymeric intrinsic HTM. This high performance is attributed to efficient hole extraction/collection (the most efficient pBBTa-BDT is highly ordered and orients π-face-down on the perovskite surface) and balanced electron/hole transport. The smooth, conformal polymer coatings suppress aerobic perovskite film degradation, significantly enhancing the solar cell 85 °C/65% RH PCE stability versus typical molecular HTMs.

New in-chain donor–acceptor semiconducting copolymers are designed and synthesized as dopant-free perovskite solar cell hole transport materials. Combining the BDT donor and the BBTa acceptor building blocks yields pBBTa-BDT copolymers with strong interchain interactions, substantial quinoidal π-character, preferential π-face-on orientation, and therefore efficient hole extraction/collection and balanced electron/hole transport. Significant enhancement of solar cell performance and environmental stability are achieved.