ZiQi Sun

Hybrid organic–inorganic halide perovskites have emerged at the forefront of solution-processable photovoltaic devices. Being the perovskite precursor mixture a complex equilibrium of species, it is very difficult to predict/control their interactions with different substrates, thus the final film properties and device performances. Here the wettability of CH₃NH₃PbI₃ (MAPbI₃) onto poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transporting layer is improved by exploiting the cooperative effect of graphene oxide (GO) and glucose inclusion. The glucose, in addition, triggers the reduction of GO, enhancing the conductivity of the PEDOT:PSS+GO+glucose based nanocomposite. The relevance of this approach toward photovoltaic applications is demonstrated by fabricating a hysteresis-free MAPbI₃ solar cells displaying a ≈37% improvement in power conversion efficiency if compared to a device grown onto pristine PEDOT:PSS. Most importantly, V_OC reaches values over 1.05 V that are among the highest ever reported for PEDOT:PSS p-i-n device architecture, suggesting minimal recombination losses, high hole-selectivity, and reduced trap density at the PEDOT:PSS along with optimized MAPbI₃ coverage.

The synergistic effect of graphene oxide and glucose in improving the conduction properties of polymer electrolyte poly(3,4-ethyl­enedioxythiophene):poly­(styrenesul­fonate) and modifying the sensible interface of perovskite solar cells is reported. This method allows obtaining hysteresis-free and high V_OC CH₃NH₃PbI₃ devices displaying a ≈37% improvement in power conversion efficiency, evidencing minimal recombination losses and very efficient charge extraction at the electrodes.

A low-bandgap (1.33 eV) Sn-based MA_0.5FA_0.5Pb_0.75Sn_0.25I₃ perovskite is developed via combined compositional, process, and interfacial engineering. It can deliver a high power conversion efficiency (PCE) of 14.19%. Finally, a four-terminal all-perovskite tandem solar cell is demonstrated by combining this low-bandgap cell with a semitransparent MAPbI₃ cell to achieve a high efficiency of 19.08%.

The radiation hardness of CH₃NH₃PbI₃-based solar cells is evaluated from in situ measurements during high-energy proton irradiation. These organic–inorganic perovskites exhibit radiation hardness and withstand proton doses that exceed the damage threshold of crystalline silicon by almost 3 orders of magnitude. Moreover, after termination of the proton irradiation, a self-healing process of the solar cells commences.

Spray-coating is a versatile coating technique that can be used to deposit functional films over large areas at speed. Here, spray-coating is used to fabricate inverted perovskite solar cell devices in which all of the solution-processible layers (PEDOT:PSS, perovskite, and PCBM) are deposited by ultrasonic spray-casting in air. Using such techniques, all-spray-cast devices having a champion power conversion efficiency (PCE) of 9.9% are fabricated. Such performance compares favorably with reference devices spin-cast under a nitrogen atmosphere that has a champion PCE of 12.8%. Losses in device efficiency are ascribed to lower surface coverage and reduced uniformity of the spray-cast perovskite layer.

Spray-coating is a versatile coating technique that can be used to deposit functional films over large areas at speed. Here, the authors fabricate inverted perovskite solar cell devices in which all of the solution-processible layers are deposited by ultrasonic spray-casting in air leading to all-spray-cast devices having a champion power conversion efficiency of 9.9%.

An organic–inorganic halide CH₃NH₃SnI₃ perovskite with significantly improved structural stability is obtained via pressure-induced amorphization and recrystallization. In situ high-pressure resistance measurements reveal an increased electrical conductivity by 300% in the pressure-treated perovskite. Photocurrent measurements also reveal a substantial enhancement in visible-light responsiveness. The mechanism underlying the enhanced properties is shown to be associated with the pressure-induced structural modification.

Controlling the morphology and surface passivation in perovskite solar cells is paramount in obtaining optimal optoelectronic properties. This study incorporates N-doped graphene nanosheets in the perovskite layer, which simultaneously induces an improved morphology and surface passivation at the perovskite/spiro interface, resulting in enhancement in all photovoltaic parameters.

The performance of hybrid perovskite-based light-emitting diodes (LEDs) is markedly enhanced by the application of a NiO_x electrode interlayer and moderate methylamine treatment. A hybrid perovskite-based LED exhibits a peak luminous efficiency of 15.9 cd A⁻¹ biased at 8.5 V, 407.65 mA cm⁻², and 65 300 cd m⁻², showing a distinctive impact for future applications.

On page 6562, K. T. Nam, H. W. Jang, and co-workers describe an ultralow electric field and high-ON/OFF-ratio resistive-switching behavior of solution-processed organo-metal halide perovskite (OHP) films with the rotational motion of the A-site molecular cation. Metal/OHP/metal cells exhibit electroforming-free resistive switching at an electric field of 3.25 × 10³ V cm⁻¹ for four distinguishable ON-state resistance levels.

On page 6695, X. Y. Gao, L.-S. Liao, and co-workers describe the fabrication of mixed Pb–In perovskite solar cells, using indium (III) chloride and lead (II) chloride with methylammonium iodide. A maximum power conversion efficiency as high as 17.55% is achieved owing to the high quality of the perovskites with multiple ordered crystal orientations. This work demonstrates the possibility of substituting the Pb (II) by using In (III), which opens a broad route to fabricating alloy perovskite solar cells with mitigated ecological impact.

Molecular orientation plays a critical role in determing the performance of all-polymer solar cells. In article number 1600504, Bumjoon J. Kim and co-workers report an approach for tuning the molecular crystallinity and orientation of naphthalenediimide-bithiophene-based n-type polymers (P(NDI2HD-T2)) by controlling their number average molecular weights. The cover image depicts the bulk heterojunction all-polymer solar cell with PTB7-Th donor and P(NDI2HD-T2) acceptor. The packing orientation of two polymers in thin film and at the interface is critical for producing high-performance solar cells.

On page 5400, M. J. Ko and co-workers report that hydrophobic ligand capped pyrite nanoparticles can act as a bi-functional hole transporting layer (HTL) (charge transporting layer and moisture-proof layer) for perovskite solar cells. Perovskite solar cells employing this HTL exhibit very stable performance under moist condition for at least 1000 hours.

A crosslinked organic hole-transporting layer (HTL) is developed to realize highly efficient and stable perovskite solar cells via a facile thiol-ene thermal reaction. This crosslinked HTL not only facilitates hole extraction from perovskites, but also functions as an effective protective barrier. A high-performance (power conversion efficiency: 18.3%) device is demonstrated to show respectable photo and thermal stability without encapsulation.

The photophysical properties and solar cell performance of the classical donor–acceptor copolymer PCDTBT

(poly(N-9′-heptadecanyl-2,7-carbazole-alt -5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole))) in relation to unintentionally formed main chain defects are investigated. Carbazole–carbazole homocouplings (Cbz hc) are found to significant extent in PCDTBT made with a variety of Suzuki polycondensation conditions. Cbz hc vary between 0 and 8 mol% depending on the synthetic protocol used, and are quantified by detailed nuclear magnetic resonance spectroscopy including model compounds, which allows to establish a calibration curve from optical spectroscopy. The results are corroborated by extended time-dependent density functional theory investigations on the structural, electronic, and optical properties of regularly alternating and homocoupled chains. The photovoltaic properties of PCDTBT:fullerene blend solar cells significantly depend on the Cbz hc content for constant molecular weight, whereby an increasing amount of Cbz hc leads to strongly decreased short circuit currents J_SC. With increasing Cbz hc content, J_SC decreases more strongly than the intensity of the low energy absorption band, suggesting that small losses in absorption cannot explain the decrease in J_SC alone, rather than combined effects of a more localized LUMO level on the TBT unit and lower hole mobilities found in highly defective samples. Homocoupling-free PCDTBT with optimized molecular weight yields the highest efficiency up to 7.2% without extensive optimization.

Electronic main chain defects in conjugated polymers caused by carbazole homocouplings (Cbz hc) significantly deteriorate the performance of organic photovoltaics as shown for poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (PCDTBT):PC₇₁BM devices. Cbz hc occur frequently, are quantified for a variety of Suzuki conditions, and are correlated with photophysical properties. PCDTBT without main chain defects and optimized molar mass gives the highest power conversion efficiencies exceeding 7%.

The development of effective and stable hole transporting materials (HTMs) is very important for achieving high-performance planar perovskite solar cells (PSCs). Herein, copper salts (cuprous thiocyanate (CuSCN) or cuprous iodide (CuI)) doped 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD) based on a solution processing as the HTM in PSCs is demonstrated. The incorporation of CuSCN (or CuI) realizes a p-type doping with efficient charge transfer complex, which results in improved film conductivity and hole mobility in spiro-OMeTAD:CuSCN (or CuI) composite films. As a result, the PCE is largely improved from 14.82% to 18.02% due to obvious enhancements in the cell parameters of short-circuit current density and fill factor. Besides the HTM role, the composite film can suppress the film aggregation and crystallization of spiro-OMeTAD films with reduced pinholes and voids, which slows down the perovskite decomposition by avoiding the moisture infiltration to some extent. The finding in this work provides a simple method to improve the efficiency and stability of planar perovskite solar cells.

Copper salts (cuprous thiocyanate or cuprous iodide) doped 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene is used as the hole transport layer in planar perovskite solar cells by their good film conductivity and hole mobility. As a result, a maximum 18.02% power conversion efficiency is achieved with improved cell stability. 2D grazing incidence X-ray diffraction technique is utilized to probe the cell degradation process.

The origin of hysteresis behavior is probed in perovskite solar cells (PSCs) with simultaneous measurements of cell open circuit voltage (V_oc) and photoluminescence intensity over time following illumination of the cell. It is shown, for the first time, that the transient changes in terminal voltage and luminescent intensity do not follow the relationship that would be predicted by the generalized Plank radiation law. A mechanism is proposed based on the presence of a resistive barrier to majority carrier flow at the interface between the perovskite film and the electron or hole transport layer, in combination with significant interface recombination. This results in a decoupling of the internal quasi-Fermi level separation and the externally measured voltage. A simple numerical model is used to provide in-principle validation for the proposed mechanism and it is confirmed that mobile ionic species are a likely candidate for creating the time-varying majority carrier bottleneck by its reduced conductivity. The findings show that the V_oc of PSCs may be lower than the limit imposed by the cell luminescence efficiency, even under steady-state conditions.

The origin of hysteresis behavior in perovskite solar cells is probed with simultaneous measurements of cell open circuit voltage and photoluminescence intensity over time following illumination of the cell. The findings demonstrate the existence of a resistive barrier to majority carrier flow at the interfaces of these devices, in combination with significant interface recombination.

All-polymer solar cells with 7.57% power conversion efficiency are achieved via a new perylenediimide-based polymeric acceptor. Furthermore, the device processed in ambient air without encapsulation can still reach a high power conversion efficiency (PCE) of 7.49%, which is a significant economic advantage from an industrial processing perspective. These results represent the highest PCE achieved from perylenediimide-based polymers.

Anisotropic charge transfer (CT) state absorption originating from the natural transition dipole alignment at planar donor–acceptor interfaces can be exploited to reveal the full CT state absorption spectrum in organic solar cells. Application of this method reveals the existence of previously unobserved, higher-lying CT absorption buried beneath the excitonic background.

By simply spinning Au nanoparticles on the surface of graphene/GaAs heterojunction, a stable solar cell with a power conversion efficiency (PCE) of 16.2% is achieved. The surface plasmon resonance of Au nanoparticles enhances the electromagnetic field near the graphene/GaAs junction area, which significantly improves the short current value and PCE.