ZiQi Sun

Organic solar cells (OSCs) made of donor/acceptor bulk-heterojunction active layers have been of widespread interest in converting sunlight to electricity. Characterizing of the complex morphology at multiple length scales of polymer:nonfullerene small molecular acceptor (SMA) systems remains largely unexplored. Through detailed characterizations (hard/soft X-ray scattering) of the record-efficiency polymer:SMA system with a close analog, quantitative morphological parameters are related to the device performance parameters and fundamental morphology–performance relationships that explain why additive use and thermal annealing are needed for optimized performance are established. A linear correlation between the average purity variations at small length scale (≈10 nm) and photovoltaic device characteristics across all processing protocols is observed in ≈12%-efficiency polymer:SMA systems. In addition, molecular interactions as reflected by the estimated Flory–Huggins interaction parameters are used to provide context of the room temperature morphology results. Comparison with results from annealed devices suggests that the two SMA systems compared show upper and lower critical solution temperature behavior, respectively. The in-depth understanding of the complex multilength scale nonfullerene OSC morphology may guide the device optimization and new materials development and indicates that thermodynamic properties of materials systems should be studied in more detail to aid in designing optimized protocols efficiently.

Quantitative morphological parameters correlate well with the photovoltaic performance of six high-efficiency nonfullerene small molecular acceptors systems. Synergistic higher π–π coherence lengths and average domain purity result in over 12% efficiency fullerene-free organic solar cell (OSC) with a sequential treatment of solvent additive and thermal annealing. The work highlights the need for more detailed studies into the thermodynamics of OSCs.

An important property of hybrid layered perovskite is the possibility to reduce its dimensionality to provide wider band gap and better stability. In this work, 2D perovskite of the structure (PEA)₂(MA)_n–1Pb_nBr_3n+1 has been sensitized, where PEA is phenyl ethyl-ammonium, MA is methyl-ammonium, and using only bromide as the halide. The number of the perovskite layers has been varied (n) from n = 1 through n = ∞. Optical and physical characterization verify the layered structure and the increase in the band gap. The photovoltaic performance shows higher open circuit voltage (V_oc) for the quasi 2D perovskite (i.e., n = 40, 50, 60) compared to the 3D perovskite. V_oc of 1.3 V without hole transport material (HTM) and V_oc of 1.46 V using HTM have been demonstrated, with corresponding efficiency of 6.3% and 8.5%, among the highest reported. The lower mobility and transport in the quasi 2D perovskites have been proved effective to gain high V_oc with high efficiency, further supported by ab initio calculations and charge extraction measurements. Bromide is the only halide used in these quasi 2D perovskites, as mixing halides have recently revealed instability of the perovskite structure. These quasi 2D materials are promising candidates for use in optoelectronic applications that simultaneously require high voltage and high efficiency.

A quasi 2D perovskite materials based on bromide, having low exciton binding energy, high absorbance coefficient, and high stability is presented. These materials provide high open circuit voltage and high power conversion efficiency in perovskite solar cells with and without hole transport material.

On page 10149, a simple and mass-scalable method, ultrafiltration, is proposed for achieving high-conductivity poly(3,4-ethylenedioxythiophene) (PEDOT): poly(4-styrenesulfonate) (PEDOT:PSS) by Y.-Y. Noh, J.-H. Kim, and co-workers. By effectively removing excess PSS and various reaction impurities using repeated 100 nm-poremembrane filtration, the purified PEDOT:PSS shows impressive conductivity and sheet resistance as high as 2000 S cm⁻¹ and 37.8 ± 4.5 Ω□⁻¹, respectively.

Layer deposition of organometal halide perovskites for solar cells usually involves tedious experimentation to establish the optimum processing conditions. Important parameters are the time and temperature of thermal annealing. Here, it is demonstrated that in situ photoluminescence allows to determine the optimal annealing procedure without fabricating complete solar cells. A deposition method is used in which dense layers of perovskite crystals are formed within seconds in ambient air by hot casting a mixture of lead acetate, lead chloride, and methylammonium iodide. The as-cast perovskite layers are highly luminescent because charge carriers are unable to reach the charge extraction layers that quench the photoluminescence. Thermal annealing enhances charge transport and quenches the photoluminescence, but deteriorates the photovoltaic performance via decomposition of the perovskite if applied for a too long time. It is demonstrated that the optimal annealing time coincides with the time required for the in situ measured photoluminescence intensity to reach its baseline value for annealing temperatures in the range of 80–100 °C. This results in efficient (>14%) perovskite solar cells and shows that in situ photoluminescence is a simple but powerful tool for in-line quality monitoring of perovskite films.

In situ photoluminescence is used to determine the optimal thermal annealing conditions of organometal halide perovskite layers, prior to completing the solar cell. The method allows fabricating efficient (>14%) planar perovskite solar cells under ambient conditions that feature good stability and low hysteresis.

Organic electronics has the potential to be incorporated in any kind of surface morphology for wearable or fully portable applications. Unfortunately when organic devices, such as solar cells, are fabricated on flexible substrates, the device performance is severely limited unless the physical properties of such substrates are carefully chosen. Here, it is demonstrated that layers of nanoparticles with a size gradient distribution can be used to obtain high performance solar cell devices that can be effectively delaminated from an original flat and rigid glass substrate. Such sacrificial nanoparticles layers are incorporated in between the glass substrate and the semitransparent electrode of a polymer:fullerene (PTB7:PC₇₁BM) cell. After the cell delamination, freestanding flexible devices with power conversion efficiencies as high as 7.12% are obtained, which corresponds to 90% of the performance of the same cell fabricated on a standard glass smooth surface.

The performance of flexible electronic devices is, in general, strongly limited by the physical properties of the specific substrate used. It is demonstrated that a sacrificial layer of multiple size nanoparticles with a size gradient distribution can be used to successfully delaminate organic solar cells from an original flat and rigid glass substrate to achieve efficient freestanding flexible devices.

Earth-abundant Cu₂BaSnS₄ (CBTS) thin films exhibit a wide bandgap of 2.04–2.07 eV, a high absorption coefficient > 10⁴ cm⁻¹, and a p-type conductivity, suitable as a top-cell absorber in tandem solar cell devices. In this work, sputtered oxygenated CdS (CdS:O) buffer layers are demonstrated to create a good p–n diode with CBTS and enable high open-circuit voltages of 0.9–1.1 V by minimizing interface recombination. The best power conversion efficiency of 2.03% is reached under AM 1.5G illumination based on the configuration of fluorine-doped SnO₂ (back contact)/CBTS/CdS:O/CdS/ZnO/aluminum-doped ZnO (front contact).

Earth-abundant Cu₂BaSnS₄ (CBTS) thin films exhibit the optoelectronic and defect properties suitable as an absorber for the top cell of tandem photovoltaic solar cell devices. Oxygenated CdS (CdS:O) buffers enable high open-circuit voltages of 0.9–1.1 V in CBTS thin-film solar cells by minimizing interface recombination.

The first homo-tandem non-fullerene organic solar cell enabled by a novel recombination layer which only requires a very mild thermal annealing treatment is reported. The best efficiency achieved is 10.8% with a V_oc over 2.1 V, which is the highest V_oc for double-junction organic solar cells reported to date.

New fiber-type piezoelectric nanogenerator devices consisting of radially aligned perovskite PbTiO₃ nanotubes are designed for energy harvesting from arbitrary mechanical motion. The free-standing fiber-type nanogenerators generate constant amount of electric power by bending or wind motion regardless of direction, thus, extending the possibility of their practical applications.