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Triplet exciton formation in neat 7,7-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′] dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh₂)₂) and blends with [6,6]-Phenyl C₇₀ butyric acid methyl ester (PC₇₀BM), with and without the selective solvent additive 1,8-diiodooctane, is investigated by means of spin sensitive photoluminescence measurements. For all three material systems, a significant amount of long living triplet excitons is detected, situated on the p-DTS(FBTTh₂)₂ molecules. The characteristic zero-field splitting parameters for this state are identified to be D = 42 mT (1177 MHz) and E = 5 mT (140 MHz). However, no triplet excitons located on PC₇₀BM are detectable. Using electrically detected spin resonance, the presence of these triplet excitons is confirmed even at room temperature, highlighting that triplet excitons form during solar cell operation and influence the photocurrent and photovoltage. Surprisingly, the superior performing blend is found to have the largest triplet population. It is concluded, that the formation of triplet excitons from charge transfer states via electron back transfer has no crucial impact on device performance in p-DTS(FBTTh₂)₂:PC₇₀BM based solar cells.

Various pathways of triplet exciton formation, such as intersystem crossing and electron back transfer (EBT), are investigated in the high-performing organic photovoltaics system 7,7-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′] dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole):[6,6]-Phenyl C₇₀ butyric acid methyl ester (PC₇₀BM). Using spin sensitive detection of photoluminescence and photocurrent in thin films and solar cells, it is concluded that the formation of triplet excitons via EBT does not have a crucial impact on the device efficiency.

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.

Poly(3-hexyl thiophene) (P3HT) can be easily synthesized, demonstrating the large potential to decrease the cost in large-scale synthesis. Thus, the development of novel non-fullerene acceptor to match well with P3HT is urgent and necessary. The first benzotriazole-containing non-fullerene acceptor is designed and synthesized, and high open-circuit voltage (V _oc) of 1.02 V, fill factor (FF) of 0.70, and power conversion efficiency (PCE) of 5.24% are realized, which are remarkably higher than that of P3HT: [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) system (V _oc = 0.59 V, FF = 0.57, and PCE = 3.34%).

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.

Acceptor alloys based on n-type small molecular and fullerene derivatives are used to fabricate the ternary solar cell. The highest performance of optimized ternary device is 10.4%, which is the highest efficiency for one donor/two acceptors-based ternary systems. Three important parameters, J_SC, V_OC, and FF, of the optimized ternary device are all higher than the binary reference devices.

Small molecule organic solar cells (SMOSCs) are at the forefront of organic solar cell research and have power conversion efficiencies that match the leading polymer:fullerene organic solar cells (>10%). However, the operating physics of SMOSCs is less understood than that of their polymer:fullerene counterparts. A stronger emphasis on understanding the working mechanisms of SMOSCs is thus required. This feature article aims to highlight methods for understanding a significant loss process in SMOSCs - charge carrier recombination - by using photo-induced transient optoelectronic techniques. These techniques make it is possible to probe the charge carrier density and lifetime in devices under working conditions. Employing these techniques alongside detailed morphological studies allows relationships between interfacial recombination processes, molecular packing and film nanomorphology to be obtained and can subsequently lead to more efficent devices being produced.

Probing the recombination dynamics in small molecule organic solar cells is vital to understand how these devices operate and how we can further optimize them. In this review, it is highlighted how carrier kinetics in these devices is affected by molecular structure, device architecture and active-layer morphology. An overview of the photo-induced techniques is also provided.

A series of wide-bandgap (WBG) copolymers with different alkyl side chains are synthesized. Among them, copolymer PBT1-EH with moderatly bulky side chains on the acceptor unit shows the best photovoltaic performance with power conversion efficiency over 10%. The results suggest that the alkyl side-chain engineering is an effective strategy to further tuning the optoelectronic properties of WBG copolymers.

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is widely used as hole injection/extraction material in organic optoelectronics. However, there still exist drawbacks for PEDOT:PSS such as low work function (WF), poor structural and electrical homogeneity. To solve these problems, methylnaphthalene sulfonate formaldehyde condensate (MNSF) is applied, which has excellent dispersion property, branched chemical structure, and low cost, as dispersant and dopant instead of linear PSS to prepare PEDOT:MNSF. The hole injection/extraction capability of PEDOT:MNSF is systematically studied in organic optoelectronic devices. PEDOT:MNSF-1:6 exhibits unexpected high device performance with a maxima current efficiency of 33.4 cd A⁻¹ in blue phosphorescent organic light-emitting diode and a power conversion efficiency of 13.1% in CH₃NH₃PbI_xCl_3−x-based inverted perovskite solar cell, respectively. Compared with PEDOT:PSS, the relatively higher efficiency of PEDOT:MNSF-1:6 is attributed mainly to its higher WF of 5.29 eV, structural and electrical homogeneity. Our research displays a promising future of MNSF as a cheap and widely available alternative of PSS. Moreover, a clear map is provided for the design of dopant for PEDOT considering the structure of dopant.

Comparing with poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), methylnaphthalene sulfonate formaldehyde condensate-doped PEDOT with higher work function, excellent homogeneity, but low conductivity exhibits higher hole injection/extraction capabilities in organic light-emitting diodes/perovskite solar cells. The semiconductivity of dopant and structural homogeneity are both key factors. The results demonstrate a panoramic prospect for the future design of dopant for PEDOT.

Properties of hole transporting layers (HTLs) and back electrode are very critical to the stability of inverted bulk heterojunction organic photovoltaic (OPV) modules. Here, various deposition methods for back electrodes and materials of HTLs are examined by applying to inverted organic solar cells with a structure of indium tin oxide/ZnO/photoactive layer/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/Ag. The experiment is performed on encapsulated modules with flexible barrier films under accelerated conditions. The OPV modules with screen-printed Ag electrodes are shown to be electrically unstable with a reduction of the current density under damp heat condition at 85 °C/85% RH. Optical images for the active layer/PEDOT:PSS interface reveal that a reaction between the solvent from the Ag electrode and the underlying layers is the major cause for the degradation. In comparison with materials of the HTLs, the PEDOT:PSS layer shows low stability compared to the MoO₃ layer under the accelerated conditions. Unusual chemical changes in the PEDOT:PSS film are observed through X-ray photoelectron spectroscopy and this is further addressed by correlating the stability of the OPV devices.

Critical impact of hole transporting layer (HTL) and back electrode on the stability of organic photovoltaics (OPVs) is discussed. The active and HTL layers are damaged by solvent of the printed Ag electrode, resulting in abrupt decay of the power conversion efficiency. Unstable features of the OPVs accompanied by chemical change of the PEDOT:PSS is further addressed with X-ray photoelectron spectroscopy (XPS).

A small molecule DRCN5T as the donor with the of 1.60 eV, and a 3D perylene diimide as electronic acceptor are used to fabricate non-fullerene all-small-molecule solar cells. Upon thermal annealing, the efficiency of the device is enhanced from 0.47% to 6.16% with a high V_oc of 1.04 V and a small energy loss of 0.56 eV.

A sequential-casting ternary method is developed to create stratified bulk heterojunction (BHJ) solar cells, in which the two BHJ layers are spin cast sequentially without the need of adopting a middle electrode and orthogonal solvents. This method is found to be particularly useful for polymers that form a mechanically alloyed morphology due to the high degree of miscibility in the blend.

Nonfullerene acceptor FDICTF (2,9-bis(2methylene-(3-(1,1-dicyanomethylene)indanone))-7,​12-​dihydro-​4,​4,​7,​7,​12,​12-​hexaoctyl-​4H-​cyclopenta[2″,​1″:5,​6;3″,​4″:5′,​6′]​diindeno[1,​2-​b:1′,​2′-​b′]dithiophene) modified by fusing the fluorene core in a precursor, yields 10.06% high power conversion efficiency, and demonstrates that the ladder and fused core backbone in A–D–A structure molecules is an effective design strategy for high-performance nonfullerene acceptors.

Fullerene-free organic solar cells show over 11% power conversion efficiency, processed by low toxic solvents. The applied donor and acceptor in the bulk heterojunction exhibit almost the same highest occupied molecular orbital level, yet exhibit very efficient charge creation.

Recent progress regarding planar heterojunctions (PHJs) is reviewed, with respect to the fundamental understanding of the photophysical processes at the donor/acceptor interfaces in organic photovoltaic devices (OPVs). The current state of OPV research is summarized and the advantages of PHJs as models for exploring the relationship between organic interfaces and device characteristics described. The preparation methods and the characterization of PHJ structures to provide key points for the appropriate handling of PHJs. Next, we describe the effects of the donor/acceptor interface on each photoelectric conversion process are reviewed by examining various PHJ systems to clarify what is currently known and not known. Finally, it is discussed how we the knowledge obtained by studies of PHJs can be used to overcome the current limits of OPV efficiency.

Heterojunctions of organic semiconductors are the most important element in organic photovoltaic devices (OPVs). The recent progress in research on planar heterojunctions is reviewed with respect to a fundamental understanding of the photophysical processes at the donor/acceptor interfaces in OPVs. This provides valuable information to overcome the current limitations regarding the performance of OPVs.

A formamidinium(FA)-based perovskite showns superior optoelectronic properties including better stability than methylammonium-based counterparts. Pure FA-perovskite-based light-emitting diodes (LEDs) with high efficiency are reported. Interestingly, the LED clearly shows a sub-bandgap emission at 1.7 V (bandgap 2.3 eV). This important discovery provides further insights of the charge transport mechanism in perovskite-based optoelectronic devices.

While it has been argued that field-dependent geminate pair recombination (GR) is important, this process is often disregarded when analyzing the recombination kinetics in bulk heterojunction organic solar cells (OSCs). To differentiate between the contributions of GR and nongeminate recombination (NGR) the authors study bilayer OSCs using either a PCDTBT-type polymer layer with a thickness from 14 to 66 nm or a 60 nm thick p-DTS(FBTTh₂)₂ layer as donor material and C₆₀ as acceptor. The authors measure JV-characteristics as a function of intensity and charge-extraction-by-linearly-increasing-voltage-type hole mobilities. The experiments have been complemented by Monte Carlo simulations. The authors find that fill factor (FF) decreases with increasing donor layer thickness (L_p) even at the lowest light intensities where geminate recombination dominates. The authors interpret this in terms of thickness dependent back diffusion of holes toward their siblings at the donor–acceptor interface that are already beyond the Langevin capture sphere rather than to charge accumulation at the donor–acceptor interface. This effect is absent in the p-DTS(FBTTh₂)₂ diode in which the hole mobility is by two orders of magnitude higher. At higher light intensities, NGR occurs as evidenced by the evolution of s-shape of the JV-curves and the concomitant additional decrease of the FF with increasing layer thickness.

Back diffusion of holes is identified to control the fill factor and thus the efficiency of bilayer organic solar cells. The competition between recombination at the donor–acceptor interface and extraction at the electrodes is studied by varying the donor layer thickness and light intensity such as to differentiate between monomolecular, i.e., geminate recombination and bimolecular, nongeminate recombination.

Intrinsic photodegradation of organic solar cells, theoretically attributed to CH bond rearrangement/breaking, remains a key commercialization barrier. This work presents, via dark electron paramagnetic resonance (EPR), the first experimental evidence for metastable C dangling bonds (DBs) formed by blue/UV irradiation of polymer:fullerene blend films in nitrogen. The DB density increases with irradiation and decreases ≈4-fold after 2 weeks in the dark. The dark EPR also shows increased densities of other spin-active sites in photodegraded polymer, fullerene, and polymer:fullerene blend films, consistent with broad electronic measurements of fundamental properties, including defect/gap state densities. The EPR and electronic measurements enable identification of defect states, whether in the polymer, fullerene, or at the donor/acceptor (D/A) interface. Importantly, the EPR results indicate that the DBs are at the D/A interface, as they were present only in the blend films. The role of polarons in interface DB formation is also discussed.

For the first time, strong electron paramagnetic resonance evidence is provided that irradiation of bulk heterojunction polymer:fullerene solar cells by blue and shorter wavelength photons (<495 nm), likely assisted by highly energized (“hot”) polarons, generates C dangling bonds at the polymer:fullerene interfaces. This photodegradation process may be one of the key obstacles to commercialization of such cells.

Multijunction solar cells promise higher power-conversion efficiency than the single-junction. With respect to two-terminal devices, an accurate measurement of the spectral response requires a delicate adjustment of the light- and voltage-biasing; otherwise it can result in artifacts in the data and thus misinterpretation of the cell properties. In this paper, the formation of measurement artifacts is analyzed by modeling the measurement process, that is, how the current–voltage characteristics of the component subcells evolve with the photoresponse to the incident spectrum. This enables the examination on the operation conditions of the subcells, offering additional information for the study of artifacts. In particular, the influence of shunt resistance, bias-light intensity, and bias voltage on the measurement is examined. Having observed the dynamics and vulnerability of the measurement, the proper ways to configure and interpret a measurement are discussed in depth. As a practical example, simulations of the measurements on a quadruple-junction thin-film silicon solar cell demonstrate that the modeling can be used to interpret eventual irregularities in the measured spectral response. The application of such tool is especially meaningful taking account of the diverse and rapid development of novel hybrid multijunction solar cells, in which the role of reliable characterizations is essential.

The formation of artifacts in the spectral response measurement of two-terminal multijunction solar cells is visually revealed by changing the cell properties and bias conditions in modeling. The simulations greatly help to understand the origins of artifacts, and to avoid the pitfall of misinterpreting the measured data, making the research in novel hybrid multijunction solar cells more reliable.

C.-Z. Li, H. Chen and co-workers present, on page 9729, for the first time, efficient non-fullerene tandem organic solar cells (OSCs) by utilizing all non-fullerene acceptor-based bulk heterojunctions as sub-cells. A high power conversion efficiency of 8.48% is achieved with an ultra-high open-circuit voltage of 1.97 V, which is the highest voltage value reported to date feasible for water splitting among the efficient tandem OSCs.

Five polymer donors with distinct chemical structures and different electronic properties are surveyed in a planar and narrow-bandgap fused-ring electron acceptor (IDIC)-based organic solar cells, which exhibit power conversion efficiencies of up to 11%.