wangzhaowei

Unreacted lead iodide is commonly believed to be beneficial to the efficiency of methylammonium lead iodide perovskite based solar cells, since it has been proposed to passivate the defects in perovskite grain boundaries. However, it is shown here that the presence of unreacted PbI₂ results in an intrinsic instability of the film under illumination, leading to the film degradation under inert atmosphere and faster degradation upon exposure to illumination and humidity. The perovskite films without lead iodide have improved stability, but lower efficiency due to inferior film morphology (smaller grain size, the presence of pinholes). Optimization of the deposition process resulted in PbI₂-free perovskite films giving comparable efficiency to those with excess PbI₂ (14.2 ± 1.3% compared to 15.1 ± 0.9%) Thus, optimization of the deposition process for PbI₂-free films leads to dense, pinhole-free, large grain size perovskite films which result in cells with high efficiency without detrimental effects on the film photostability caused by excess PbI₂. However, it should be noted that for encapsulated devices illuminated through the substrate (fluorine-doped tin oxide glass, TiO₂ film), film photostability is not a key factor in the device degradation.

PbI₂ residue-containing perovskite films are intrinsically unstable under illumination. While the excess PbI₂ improves the cell efficiency due to improved perovskite layer morphology, optimization of the deposition leads to lead iodide-free films with improved photostability and high efficiency. However, compared to photodegradation, other processes are more important in device degradation.

Lead (II) perovskite (PV, e.g., CH₃NH₃PbI_3–xCl_x) with 3 wt% polyvinylpyrrolidone (PVP) produces an ≈90 nm film containing a PVP–PV composite consisting of ≈15-nm-sized PV nanocrystals. Solar cells using 3 wt% PVP–PV have several advantages over a device using nondoped PV, which include an increase in transparency, stability of the film, efficiency, and reproducibility.

Inverted organic solar cells generally exhibit a strong s-shaped kink in the current–voltage characteristics (JV curve) that may be removed by exposure to UV light (light-soaking) leading to a drastically improved performance. Using in-device characterization methods the origin of the light-soaking issue in inverted solar cells employing titanium dioxide (TiO₂) as an electron selective layer is clarified. An injected hole reservoir accumulated at the TiO₂/organic interface of the pristine device is observed from extraction current transients; the hole reservoir increases the recombination and results in an s-shape in the JV curve of pristine devices. The hole reservoir and the s-shape is a result of the energetics at the selective contact in the pristine device; the effect of UV exposure is to decrease the work function of the indium tin oxide/TiO₂-contact, increasing the built-in potential. This hinders the build-up of the hole reservoir and the s-shape is removed. The proposed model is in excellent agreement with drift-diffusion simulations.

Using in-device characterization methods and drift-diffusion simulations, the origin of the s-shaped JV curve and the light-soaking issue in inverted solar cells employing TiO₂ as an electron-selective layer are clarified. The s-shape is a result of the energetics at the selective contact in the pristine device; the effect of UV exposure is to decrease the work function of the ITO/TiO₂-contact.

How free charge is generated at organic donor–acceptor interfaces is an important question, as the binding energy of the lowest energy (localized) charge transfer states should be too high for the electron and hole to escape each other. Recently, it has been proposed that delocalization of the electronic states participating in charge transfer is crucial, and aggregated or otherwise locally ordered structures of the donor or the acceptor are the precondition for this electronic characteristic. The effect of intermolecular aggregation of both the polymer donor and fullerene acceptor on charge separation is studied. In the first case, the dilute electron acceptor triethylsilylhydroxy-1,4,8,11,15,18,22,25-octabutoxyphthalocyaninatosilicon(IV) (SiPc) is used to eliminate the influence of acceptor aggregation, and control polymer order through side-chain regioregularity, comparing charge generation in 96% regioregular (RR-) poly(3-hexylthiophene) (P3HT) with its regiorandom (RRa-) counterpart. In the second case, ordered phases in the polymer are eliminated by using RRa-P3HT, and phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) is used as the acceptor, varying its concentration to control aggregation. Time-resolved microwave conductivity, time-resolved photoluminescence, and transient absorption spectroscopy measurements show that while ultrafast charge transfer occurs in all samples, long-lived charge carriers are only produced in films with intermolecular aggregates of either RR-P3HT or PC₆₁BM, and that polymer aggregates are just as effective in this regard as those of fullerenes.

The presence of aggregate/ordered phases in P3HT/PCBM samples controls the branching between bound geminate pairs and free charge. Both polymer crystallites and fullerene aggregates can facilitate free charge generation.

In article number 1501580, Eva Herzig and co-workers use time-resolved scattering measurements to reveal the complete solidification process inside the photoactive layer of an organic solar cell. Using an industrial slot-die coater integrated into a synchrotron beamline the sample can be continuously illuminated and therefore probed in-situ with x-rays while printing. Aggregation and crystallization processes can be tracked to reveal the structure-function relationships in the final thin film.

Ternary structures are demonstrated as a promising approach to increase the efficiency and light harvesting of solar cells. A high power conversion efficiency of 10.2% is achieved for ternary organic solar cells with two efficient polymer donors. The improved performance is attributed to the synergistic effects of enhanced light absorption and charge transport, efficient energy transfer, improved charge generation and morphology.

Trap-assisted recombination represents one of the main loss mechanisms in bulk-heterojunction (BHJ) solar cells. In article number 1501527, Nicola Gasparini, Tayebeh Ameri and co-workers show that introducing a near infrared (NIR) sensitizer into a donor-acceptor host system suppresses trap-assisted recombination in the binary blend, leading to enhanced efficiency at very low light levels. This approach represents a promising material strategy for indoor and versatile outdoor applications. Cover design by George Spyropoulos.

Metal nanoparticles (NPs) improve performance and stability by enhancing the internal quantum efficiency (IQE) of organic solar cells, as presented by Jeong Young Park, Jung-Yong Lee and co-workers in article number 1501393. An IQE level of approximately 100% is achieved by embedding the AgNPs in an electron-transporting layer with an inverted structure. The AgNPs in the polyethylenimine-ethoxylated (PEIE) layer facilitate charge carrier generation and transport, and improve the extraction efficiency, leading to a power conversion efficiency of 10.1%.

Two chemically tailored new conjugated copolymers, HSL1 and HSL2, were developed and applied as hole selective layers to improve the anode interface of fullerene/perovskite planar heterojunction solar cells. The introduction of polar functional groups on the polymer side chains increases the surface energy of the hole selective layers (HSLs), which promote better wetting with the perovskite films and lead to better films with full coverage and high crystallinity. The deep highest occupied molecular orbital levels of the HSLs align well with the valence band of the perovskite semiconductors, resulted in increase photovoltage. The high lying lowest unoccupied molecule orbital level provides sufficient electron blocking ability to prevent electrons from reaching the anode and reduces the interfacial trap-assisted recombination at the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/perovskite interface, resulting in a longer charge-recombination lifetime and shorter charge-extraction time. In the presence of the HSLs, high-performance CH₃NH₃PbI _x Cl_{3− x} perovskite solar cells with a power conversion efficiency (PCE) of 16.6% (V _oc: 1.07 V) and CH₃NH₃Pb(I_0.3Br_0.7) _x Cl_{3− x} cells with a PCE of 10.3% (V _oc: 1.34 V) can be realized.

The perovskite photovoltaic voltage output can be modulated by incorporating two new alcohol-soluble polymeric hole selective layers (HSLs). The deep highest occupied molecular orbital level and high lying lowest unoccupied molecular orbital level of HSLs can effectively increase open-circut voltage and reduce charge recombination loss, resulting in high performance perovskite solar cells with improved power conversion efficiencies.