Liuyanfeng

In this study the thickness of the PTB7-Th:PC₇₁BM bulk heterojunction (BHJ) film and the PF3N-2TNDI electron transport layer (ETL) is systematically tuned to achieve polymer solar cells (PSCs) with optimized power conversion efficiency (PCE) of over 9% when an ultrathin BHJ of 50 nm is used. Optical modeling suggests that the high PCE is attributed to the optical spacer effect from the ETL, which not only maximizes the optical field within the BHJ film but also facilitates the formation of a more homogeneously distributed charge generation profile across the BHJ film. Experimentally it is further proved that the extra photocurrent produced at the PTB7-Th/PF3N-2TNDI interface also contributes to the improved performance. Taking advantage of this high performance thin film device structure, one step further is taken to fabricate semitransparent PSCs (ST-PSCs) by using an ultrathin transparent Ag cathode to replace the thick Ag mirror cathode, yielding a series of high performance ST-PSCs with PCEs over 6% and average visible transmittance between 20% and 30%. These ST-PSCs also possess remarkable transparency color perception and rendering properties, which are state-of-the-art and fulfill the performance criteria for potential use as power-generating windows in near future.

An efficient electron transport layer of PF3N-2TNDI is introduced to improve the performance of PTB7-Th:PC₇₁BM based semitransparent polymer solar cells (ST-PSCs). PF3N-2TNDI can facilitate extra photocurrent generation and promote formation of high quality ultrathin Ag transparent cathode. These combined effects eventually lead to a new performance record of 6% power conversion efficiency with the corresponding average visible transmittance of ≈30% for the polymer:fullerene based ST-PSCs, and remarkable transparency color perception and rendering properties are also realized.

Functionalized imidazolium iodide salts (ionic liquids) modified with CH₂CHCH₂, CH₂CCH, or CH₂CN groups are applied as dopants in the synthesis of CH₃NH₃PbI₃-type perovskites together with a fumigation step. Notably, a solar cell device prepared from the perovskite film doped with the salt containing the CH₂CHCH₂ side-chain has a power conversion efficiency of 19.21%, which is the highest efficiency reported for perovskite solar cells involving a fumigation step. However, doping with the imidazolium salts with the CH₂CCH and CH₂CN groups result in perovskite layers that lead to solar cell devices with similar or lower power conversion efficiencies than the dopant-free cell.

Perovskite films, grown from PbI₂:MAI in DMSO in the presence of functionalized ionic-liquid (imidazolium iodide) dopants and incorporated into perovskite solar cells, are reported. One cell has a power conversion efficiency exceeding 19%. Difference in power conversion efficiency can be traced to the physical properties of imidazolium-PbI₃ salts that form during the preparation of the film.

Despite rapid advances in the field of nonfullerene polymer solar cells (NF-PSCs), successful examples of random polymer-based NF-PSCs are limited. In this study, it is demonstrated that random donor polymers based on thieno[2′,3′:5′,6′]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione (TPTI) containing two simple thiophene (T) and bithiophene (2T) electron-rich moieties (PTTI-Tx) can be promising materials for the fabrication of highly efficient NF-PSCs. With negligible influence on optical bandgaps and energy levels, the crystalline behavior of PTTI-Tx polymers was modulated by varying the T:2T ratio in the polymer backbone; this resulted in the formation of different microstructures upon blending with a nonfullerene m-ITIC acceptor in NF-PSCs. In particular, a PTPTI-T70:m-ITIC system enabled favorable small-scale phase separation with an increased population of face-on oriented crystallites, thereby boosting the processes of effective exciton dissociation and charge transport in the device. Consequently, the highest power conversion efficiency of 11.02% with an enhanced short-circuit current density of 17.12 mA cm⁻² is achieved for the random polymer-based NF-PSCs thus far. These results indicate that random terpolymerization is a simple and practical approach for the optimization of a donor polymer toward highly efficient NF-PSCs.

Over 11% efficiency random polymer-based nonfullerene solar cell is realized on the donor family of PTPTI-Tx containing various thiophene/bithiophene ratios in the backbone. A small-scale phase separation with an increased fraction of face-on oriented crystallites observed in the PTPTI-T70:m-ITIC blend enables efficient exciton dissociation and charge transport, thereby inducing a remarkably enhanced J_SC of 17.12 mA cm⁻² through this system.

“Nonfullerene” acceptors are proving effective in bulk heterojunction (BHJ) solar cells when paired with selected polymer donors. However, the principles that guide the selection of adequate polymer donors for high-efficiency BHJ solar cells with nonfullerene acceptors remain a matter of some debate and, while polymer main-chain substitutions may have a direct influence on the donor–acceptor interplay, those effects should be examined and correlated with BHJ device performance patterns. This report examines a set of wide-bandgap polymer donor analogues composed of benzo[1,2-b:4,5-b′]dithiophene (BDT), and thienyl ([2H]T) or 3,4-difluorothiophene ([2F]T) motifs, and their BHJ device performance pattern with the nonfullerene acceptor “ITIC”. Studies show that the fluorine- and ring-substituted derivative PBDT(T)[2F]T largely outperforms its other two polymer donor counterparts, reaching power conversion efficiencies as high as 9.8%. Combining several characterization techniques, the gradual device performance improvements observed on swapping PBDT[2H]T for PBDT[2F]T, and then for PBDT(T)[2F]T, are found to result from (i) notably improved charge generation and collection efficiencies (estimated as ≈60%, 80%, and 90%, respectively), and (ii) reduced geminate recombination (being suppressed from ≈30%, 25% to 10%) and bimolecular recombination (inferred from recombination rate constant comparisons). These examinations will have broader implications for further studies on the optimization of BHJ solar cell efficiencies with polymer donors and a wider range of nonfullerene acceptors.

Swapping main-chain substituents in a set of analogous wide-bandgap polymer donors is shown to result in gradual bulk-heterojunction (BHJ) device performance improvements when the polymers are combined with the nonfullerene acceptor “ITIC”. The gradual improvements result from better charge generation, collection, and reduced geminate and bimolecular recombination, leading to polymer-nonfullerene BHJ solar cells with power conversion efficiencies as high as 9.8%.

The development of organometal halide perovskite solar cells has grown rapidly and the highest efficiency of the devices has recently surpassed 22%. Because these solar cells contain toxic lead, a sustainable strategy is required to prevent environmental pollution and avoid healthy hazard caused by possible lead outflow. Here, in situ recycling PbI₂ from thermal decomposition CH₃NH₃PbI₃ perovskite films for efficient perovskite solar cells was developed. The thermal behavior of CH₃NH₃PbI₃ perovskite and its individual components were examined by thermogravimetric analysis. By optimizing the process of thermal decomposition CH₃NH₃PbI₃ film, the complete conversion from CH₃NH₃PbI₃ to pure PbI₂ layer with a mesoporous scaffold was achieved. The mesoporous structure readily promotes the conversion efficiency of perovskite and consequently results in high-performance device. A perovskite crystal growth mechanism on the mesoporous PbI₂ structure was proposed. These results suggest that in situ recycled PbI₂ scaffolds can be a new route in manipulating the morphology of the perovskite active layer, providing new possibilities for high performance. Meanwhile, the risk of lead outflow can be released, and the saving-energy fabrication of efficient solar cells can be realized.

This is a manuscript by Jie Xu, Ziyang Hu*, Like Huang, Xiaokun Huang, Xianyu Jia, Jing Zhang, Jianjun Zhang, and Yuejin Zhu and titled “In Situ Recycle of PbI₂ as a Step Towards Sustainable Perovskite Solar Cells.” In this paper, high-efficiency perovskite solar cells using in situ recycling PbI₂ from thermal decomposition perovskite were developed, demonstrating a process to address the environmental and health issues of lead-based perovskite cells, which offers a step towards sustainable solar technology.

Semitransparent solar cells can provide not only efficient power-generation but also appealing images and show promising applications in building integrated photovoltaics, wearable electronics, photovoltaic vehicles and so forth in the future. Such devices have been successfully realized by incorporating transparent electrodes in new generation low-cost solar cells, including organic solar cells (OSCs), dye-sensitized solar cells (DSCs) and organometal halide perovskite solar cells (PSCs). In this review, the advances in the preparation of semitransparent OSCs, DSCs, and PSCs are summarized, focusing on the top transparent electrode materials and device designs, which are all crucial to the performance of these devices. Techniques for optimizing the efficiency, color and transparency of the devices are addressed in detail. Finally, a summary of the research field and an outlook into the future development in this area are provided.

Recent developments of semitransparent organic solar cells, dye-sensitized solar cells, and perovskite solar cells are reviewed with a focus on different device design, transparent top electrode materials, and the corresponding device fabrication techniques. Key issues related to the optimization of the efficiency, color, and transparency of the semitransparent photovoltaic devices are discussed in detail.

The π-conjugated organic small molecule 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl) benzenamine] (TAPC) has been explored as an efficient hole transport material to replace poly(3,4-ethylenedio-xythiophene):poly(styrenesulfonate) (PEDOT:PSS) in the preparation of p-i-n type CH₃NH₃PbI₃ perovskite solar cells. Smooth, uniform, and hydrophobic TAPC hole transport layers can be facilely deposited through solution casting without the need for any dopants. The power conversion efficiency of perovskite solar cells shows very weak TAPC layer thickness dependence across the range from 5 to 90 nm. Thermal annealing enables improved hole conductivity and efficient charge transport through an increase in TAPC crystallinity. The perovskite photoactive layer cast onto thermally annealed TAPC displays large grains and low residual PbI₂, leading to a high charge recombination resistance. After optimization, a stabilized power conversion efficiency of 18.80% is achieved with marginal hysteresis, much higher than the value of 12.90% achieved using PEDOT:PSS. The TAPC-based devices also demonstrate superior stability compared with the PEDOT:PSS-based devices when stored in ambient circumstances, with a relatively high humidity ranging from 50 to 85%.

Conjugated molecule 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl) benzenamine] (TAPC) has been explored to replace poly(3,4-ethylenedio-xythiophene):poly(styrenesulfonate) in perovskite solar cells. The CH₃NH₃PbI₃ solar cells are hysteresis-free, with marginal dependence on the thickness of TAPC, and achieve a power conversion efficiency of 18.8 over 12.9% as a result of increased J_sc, V_oc, and fill factor.

Two regioisomeric D₁-A-D-A-D₁ type π-conjugated molecules (1,4-bis{5-[4-(5-fluoro-7-(5-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole)]­thiophen-2-yl}-2,5-bis(hexyldecyloxy)benzene (Prox-FBT) and 1,4-bis{5-[4-(6-fluoro-7-(5-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole)]­thiophen-2-yl}-2,5-bis(hexyldecyloxy)benzene (Dis-FBT)) are synthesized, by controlling the fluorine topology to be proximal or distal relative to the central core. The different F geometries are confirmed by the ¹H–¹H nuclear Overhauer effect spectroscopy (NOESY). Clearly different optical, electrochemical, and thermal transition behaviors are obtained, i.e., stronger absorption, deeper valance band (by ≈0.2 eV), and higher melting/recrystallization temperatures (by 7–20 °C) are observed for Dis-FBT. The different intermolecular packing and unit cell structures are also calculated for the two regioisomers, based on the powder X-ray diffraction and 2D grazing-incidence wide-angle X-ray diffraction measurements. A tighter π–π packing with a preferential monoclinic face-on orientation is extracted for Dis-FBT, compared to Prox-FBT with bimodal orientations. Different topological structures significantly affect the electrical and photovoltaic properties, where Prox-FBT shows higher parallel hole mobility (2.3 × 10⁻³ cm² V⁻¹ s⁻¹), but Dis-FBT demonstrates higher power conversion efficiency (5.47%) with a larger open-circuit voltage of 0.95 V (vs 0.79 V for Prox-FBT). The findings suggest that small changes in the topological geometry can affect the electronic structure as well as self-assembly behaviors, which can possibly be utilized for fine-adjusting the electrical properties and further optimization of optoelectronic devices.

Two regioisomeric π-conjugated mole­cules (1,4-bis{5-[4-(5-fluoro-7-(5-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole)]thiophen-2-yl}-2,5-bis(hexyldecyloxy)benzene and 1,4-bis{5-[4-(6-fluoro-7-(5-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole)]­thiophen-2-yl}-2,5-bis(hexyldecyloxy)benzene) with different fluorine topologies (referred to as proximal or distal relative to central core) are synthesized, and the correlation between the topological geometry of fluorine atoms and optoelectronic property is examined in terms of the molecular structure, intermolecular interactions, and the resulting bulk morphology.

Molecularly engineered novel dopant-free hole-transporting materials for perovskite solar cells (PSCs) combined with mixed-perovskite (FAPbI₃)_0.85(MAPbBr₃)_0.15 (MA: CH₃NH₃⁺, FA: NH=CHNH₃⁺) that exhibit an excellent power conversion efficiency of 18.9% under AM 1.5 conditions are investigated. The mobilities of FA-CN, and TPA-CN are determined to be 1.2 × 10⁻⁴ cm² V⁻¹ s⁻¹ and 1.1 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively. Exceptional stability up to 500 h is measured with the PSC based on FA-CN. Additionally, it is found that the maximum power output collected after 1300 h remained 65% of its initial value. This opens up new avenue for efficient and stable PSCs exploring new materials as alternatives to Spiro-OMeTAD.

Novel dopant-free hole-transporting materials for perovskite solar cells (PSCs), which exhibit an excellent power conversion efficiency of 18.9% under AM 1.5 conditions are investigated. The PSC based on FA-CN shows exceptional stability up to 500 h. The PCE collected during 1300 h is observed to remain at 65% of its initial value. This opens an avenue for efficient and stable PSCs exploring new materials.

A side-chain conjugation strategy in the design of nonfullerene electron acceptors is proposed, with the design and synthesis of a side-chain-conjugated acceptor (ITIC2) based on a 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]di(cyclopenta-dithiophene) electron-donating core and 1,1-dicyanomethylene-3-indanone electron-withdrawing end groups. ITIC2 with the conjugated side chains exhibits an absorption peak at 714 nm, which redshifts 12 nm relative to ITIC1. The absorption extinction coefficient of ITIC2 is 2.7 × 10⁵m⁻¹ cm⁻¹, higher than that of ITIC1 (1.5 × 10⁵m⁻¹ cm⁻¹). ITIC2 exhibits slightly higher highest occupied molecular orbital (HOMO) (−5.43 eV) and lowest unoccupied molecular orbital (LUMO) (−3.80 eV) energy levels relative to ITIC1 (HOMO: −5.48 eV; LUMO: −3.84 eV), and higher electron mobility (1.3 × 10⁻³ cm² V⁻¹ s⁻¹) than that of ITIC1 (9.6 × 10⁻⁴ cm² V⁻¹ s⁻¹). The power conversion efficiency of ITIC2-based organic solar cells is 11.0%, much higher than that of ITIC1-based control devices (8.54%). Our results demonstrate that side-chain conjugation can tune energy levels, enhance absorption, and electron mobility, and finally enhance photovoltaic performance of nonfullerene acceptors.

A side-chain conjugation strategy in the design of nonfullerene electron acceptors is proposed and the first example of a side-chain-conjugated fused-ring electron acceptor is presented. Polymer solar cells based on side-chain-conjugated ITIC2 show a champion power conversion efficiency of 11.0%, much higher than its counterpart ITIC1-based devices (8.54%).

Organic–inorganic hybrid perovskite materials exhibit a variety of physical properties. Pronounced coupling between phonon, organic cations, and the inorganic framework suggest that these materials exhibit strong light–matter interactions. The photoinduced strain of CH₃NH₃PbBr₃ is investigated using high-resolution and contactless in situ Raman spectroscopy. Under illumination, the material exhibits large blue shifts in its Raman spectra that indicate significant structural deformations (i.e., photostriction). From these shifts, the photostrictive coefficient of CH₃NH₃PbBr₃ is calculated as 2.08 × 10⁻⁸ m² W⁻¹ at room temperature under visible light illumination. The significant photostriction of CH₃NH₃PbBr₃ is attributed to a combination of the photovoltaic effect and translational symmetry loss of the molecular configuration via strong translation–rotation coupling. Unlike CH₃NH₃PbI₃, it is noted that the photostriction of CH₃NH₃PbBr₃ is extremely stable, demonstrating no signs of optical decay for at least 30 d. These results suggest the potential of CH₃NH₃PbBr₃ for applications in next-generation optical micro-electromechanical devices.

The photoinduced strain of CH₃NH₃PbBr₃ is investigated using high-resolution and contactless in situ Raman spectroscopy. Under illumination, the material exhibits large blue shifts in its Raman spectra that indicate significant structural deformations. The significant photostriction of CH₃NH₃PbBr₃ can be attributed to a combination of the photovoltaic effect and translational symmetry loss of the molecular configuration via strong translation–rotation coupling.

In this work, a fully tin-based, mixed-organic-cation perovskite absorber (FA)_x(MA)_1−xSnI₃ (FA = NH₂CH = NH₂⁺, MA = CH₃NH₃⁺) for lead-free perovskite solar cells (PSCs) with inverted structure is presented. By optimizing the ratio of FA and MA cations, a maximum power conversion efficiency of 8.12% is achieved for the (FA)_0.75(MA)_0.25SnI₃-based device along with a high open-circuit voltage of 0.61 V, which originates from improved perovskite film morphology and inhibits recombination process in the device. The cation-mixing approach proves to be a facile method for the efficiency enhancement of tin-based PSCs.

For the first time, an efficiency of over 8% is achieved for tin-based perovskite solar cells along with a high open-circuit voltage of 0.61 V by utilizing (FA)_0.75(MA)_0.25SnI₃ as the absorber. The cation-mixing method is proven to effectively improve the morphology of tin-based perovskite films and reduce recombination process in the devices.

Polymer solar cells have attracted tremendous interest in the highly competitive solar energy sector, due to the practical advantages they exhibit, such as being lightweight, flexible, and low cost, in stark contrast to traditional photovoltaic technologies. However, their successful commercialization is still hindered by issues related to device instability. Here, atomic layer deposition (ALD) is employed to deposit conformal ultrathin dielectrics, such as alumina (Al₂O₃) and zirconia (ZrO₂), on top of ZnO electron extraction layers to address problems that arise from the defect-rich nature of these layers. The deposition of dielectrics on ZnO significantly improves its interfacial electronic properties, manifested primarily with the decrease in the work function of ZnO and the concomitant reduction of the electron extraction barrier as well as the reduced recombination losses. Significant efficiency enhancement is obtained with the incorporation of six ALD cycles of Al₂O₃ into inverted devices, using photoactive layers, that consist of poly(3-hexylthiophene):indene-C₆₀-bisadduct or poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b] thiophenediyl}):[6,6]-phenyl-C₇₀-butyric acid methyl ester. More importantly, upon performing lifetime studies (over a period of 350 h), a strong improvement in polymer solar cell stability is observed when using the ALD-modified ZnO films.

Polymer solar cell technology is still under development in the key area of stability. An ultrathin atomic-layer-deposited dielectric oxide layer is inserted into inverted architecture polymer solar cells in order to address the issues that arise from the defect-rich nature of ZnO electron extraction material. Considerable improvement of the device efficiency and stability is observed.

Light management holds great promise of realizing high-performance perovskite solar cells by improving the sunlight absorption with lower recombination current and thus higher power conversion efficiency (PCE). Here, a convenient and scalable light trapping scheme is demonstrated by incorporating bioinspired moth-eye nanostructures into the metal back electrode via soft imprinting technique to enhance the light harvesting in organic–inorganic lead halide perovskite solar cells. Compared to the flat reference cell with a methylammonium lead halide perovskite (CH₃NH₃PbI_3−xCl_x) absorber, 14.3% of short-circuit current improvement is achieved for the patterned devices with moth-eye nanostructures, yielding an increased PCE up to 16.31% without sacrificing the open-circuit voltage and fill factor. The experimental and theoretical characterizations verify that the cell performance enhancement is mainly ascribed by the broadband polarization-insensitive light scattering and surface plasmonic effects due to the patterned metal back electrode. It is noteworthy that this light trapping strategy is fully compatible with solution-processed perovskite solar cells and opens up many opportunities toward the future photovoltaic applications.

A convenient and scalable light trapping scheme is demonstrated to enhance the light harvesting in organic–inorganic lead halide perovskite solar cells, which is realized by incorporating bioinspired moth-eye nanostructures into the metal back electrode via soft imprinting technique. The efficiency is enhanced to 16.3% due to self-enhanced absorption by broadband polarization-insensitive light scattering and surface plasmonic effect.

Over the past few years, hybrid halide perovskites have emerged as a highly promising class of materials for photovoltaic technology, and the power conversion efficiency of perovskite solar cells (PSCs) has accelerated at an unprecedented pace, reaching a record value of over 22%. In the context of PSC research, wide-bandgap semiconducting metal oxides have been extensively studied because of their exceptional performance for injection and extraction of photo-generated carriers. In this comprehensive review, we focus on the synthesis and applications of metal oxides as electron and hole transporters in efficient PSCs with both mesoporous and planar architectures. Metal oxides and their doped variants with proper energy band alignment with halide perovskites, in the form of nanostructured layers and compact thin films, can not only assist with charge transport but also improve the stability of PSCs under ambient conditions. Strategies for the implementation of metal oxides with tailored compositions and structures, and for the engineering of their interfaces with perovskites will be critical for the future development and commercialization of PSCs.

Hybrid perovskites are emerging as promising materials for low-cost photovoltaic technologies with high performance. Wide-bandgap metal oxides in the forms of nanostructures and compact thin films have been extensively applied as electron and hole transporters in perovskite solar cells. This review elucidates their crucial role in assisting perovskite solar cells to achieve optimal performance and stability.

In organic solar cells continuous donor and acceptor networks are considered necessary for charge extraction, whereas discontinuous neat phases and molecularly mixed donor–acceptor phases are generally regarded as detrimental. However, the impact of different levels of domain continuity, purity, and donor–acceptor mixing on charge transport remains only semiquantitatively described. Here, cosublimed donor–acceptor mixtures, where the distance between the donor sites is varied in a controlled manner from homogeneously diluted donor sites to a continuous donor network are studied. Using transient measurements, spanning from sub-picoseconds to microseconds photogenerated charge motion is measured in complete photovoltaic devices, to show that even highly diluted donor sites (5.7%–10% molar) in a buckminsterfullerene matrix enable hole transport. Hopping between isolated donor sites can occur by long-range hole tunneling through several buckminsterfullerene molecules, over distances of up to ≈4 nm. Hence, these results question the relevance of “pristine” phases and whether a continuous interpenetrating donor–acceptor network is the ideal morphology for charge transport.

Transient measurements reveal that in organic solar cells a continuous donor network is not strictly necessary for hole transport. Hole hopping between isolated donor sites can occur by long-range hole tunneling through several buckminsterfullerene molecules (4 nm). This often disregarded mechanism questions the importance of pristine phases and whether a continuous donor–acceptor network is the ideal morphology for charge transport.

Developing novel materials that tolerate thickness variations of the active layer is critical to further enhance the efficiency of polymer solar cells and enable large-scale manufacturing. Presently, only a few polymers afford high efficiencies at active layer thickness exceeding 200 nm and molecular design guidelines for developing successful materials are lacking. It is thus highly desirable to identify structural factors that determine the performance of semiconducting conjugated polymers in thick-film polymer solar cells. Here, it is demonstrated that thiophene rings, introduced in the backbone of alternating donor–acceptor type conjugated polymers, enhance the fill factor and overall efficiency for thick (>200 nm) solar cells. For a series of fluorinated semiconducting polymers derived from electron-rich benzo[1,2-b:4,5-b′]dithiophene units and electron-deficient 5,6-difluorobenzo[2,1,3]thiazole units a steady increase of the fill factor and power conversion efficiency is found when introducing thiophene rings between the donor and acceptor units. The increased performance is a synergistic result of an enhanced hole mobility and a suppressed bimolecular charge recombination, which is attributed to more favorable polymer chain packing and finer phase separation.

Introducing additional thiophene rings in conjugated polymers increases the fill factor and power conversion efficiency of polymer:fullerene solar cells with thick active layers. This “thiophene ring effect” is a synergistic result of enhanced hole mobility and suppressed bimolecular charge recombination via the formation of more favorable polymer chain packing and finer phase separation.

In this work, a highly efficient parallel connected tandem solar cell utilizing a nonfullerene acceptor is demonstrated. Guided by optical simulation, each of the active layer thicknesses of subcells are tuned to maximize its light trapping without spending intense effort to match photocurrent. Interestingly, a strong optical microcavity with dual oscillation centers is formed in a back subcell, which further enhances light absorption. The parallel tandem device shows an improved photon-to-electron response over the range between 450 and 800 nm, and a high short-circuit current density (J _SC) of 17.92 mA cm⁻². In addition, the subcells show high fill factors due to reduced recombination loss under diluted light intensity. These merits enable an overall power conversion efficiency (PCE) of >10% for this tandem cell, which represents a ≈15% enhancement compared to the optimal single-junction device. Further application of the designed parallel tandem configuration to more efficient single-junction cells enable a PCE of >11%, which is the highest efficiency among all parallel connected organic solar cells (OSCs). This work stresses the importance of employing a parallel tandem configuration for achieving efficient light harvesting in nonfullerene-based OSCs. It provides a useful strategy for exploring the ultimate performance of organic solar cells.

High-efficiency nonfullerene solar cells are demonstrated with a parallel tandem structure. Compared to the single-junction cell, significantly improved power conversion efficiency is achieved owing to enhanced light trapping and reduced charge recombination with diluted light intensity distribution. The champion cell efficiency over 11% represents the highest among all reported organic parallel tandem cells.

In the present study, random current fluctuations measured at different temperatures and for different illumination levels are used to understand the charge carrier kinetics in methylammonium lead iodide CH₃NH₃PbI₃-based perovskite solar cells. A model, combining trapping/detrapping, recombination mechanisms, and electron–phonon scattering, is formulated evidencing how the presence of shallow and deeper band tail states influences the solar cell recombination losses. At low temperatures, the observed cascade capture process indicates that the trapping of the charge carriers by shallow defects is phonon assisted directly followed by their recombination. By increasing the temperature, a phase modification of the CH₃NH₃PbI₃ absorber layer occurs and for temperatures above the phase transition at about 160 K the capture of the charge carrier takes place in two steps. The electron is first captured by a shallow defect and then it can be either emitted or thermalize down to a deeper band tail state and recombines subsequently. This result reveals that in perovskite solar cells the recombination kinetics is strongly influenced by the electron–phonon interactions. A clear correlation between the morphological structure of the perovskite grains, the energy disorder of the defect states, and the device performance is demonstrated.

A clear correlation between the morphological structure of the CH₃NH₃PbI₃ grains, the energy disorder of defect states, and the device parameters in the perovskite solar cell is shown. The recombination kinetics and the charge carrier transport are strongly influenced by the electron–phonon interactions.

After intense research and development organic solar cells have matured among the family of thin-film photovoltaic technologies. On the laboratory scale they reach power conversion efficiencies in excess of 10%. Together with other attractive features, like transparency or the compatibility with low-cost, large area processing, they open reasonable perspectives for their commercialization. However, in order to close the gap to established inorganic technologies, primarily crystalline silicon, the fundamental understanding of loss processes has to be improved. First and foremost, this concerns the energy loss between the optical gap for light absorption and the open-circuit voltage of the cell. Here, the scientific background for the different mechanisms of energy losses in organic photovoltaic cells together with current approaches toward their reduction is presented.

The energy loss between the optical gap and the open-circuit-voltage is one of the primary reasons why the efficiency of organic photovoltaic cells lags behind their inorganic counterparts. This research news highlights the scientific background and presents strategies to improve on this issue.

Hybrid metal halide perovskites have become one of the hottest topics in optoelectronic materials research in recent years. Not only have they surpassed everyone's expectations and achieved similar performance as tried and true polycrystalline silicon photovoltaic devices, but they are also finding applications in a variety of different fields, including lighting. The main advantages of hybrid metal halide perovskites are simple processability, compatible with large-scale solution processing such as roll-to-roll printing, and abundance of ingredients, all coupled to materials properties reminiscent of GaAs. On the road to this remarkable success, a series of challenges have been overcome, while some still remain. In this review, some of these challenges and possible solutions are described. In particular, understanding of the perovskite crystallization process and how this knowledge can be harnessed to enable better performing devices, how to overcome reproducibility issues and mitigate hysteresis, and the long-term prospects of the technology in terms of stability and sustainability will all be discussed.

This review of perovskite solar cells discusses the current understanding of the perovskite crystallization process, and how this knowledge can be harnessed to enable better performing devices; how to overcome reproducibility issues and mitigate hysteresis; and the long-term prospects of perovskite solar cell technology in terms of stability, cost, and sustainability.

A fused hexacyclic electron acceptor, IHIC, based on strong electron-donating group dithienocyclopentathieno[3,2-b]thiophene flanked by strong electron-withdrawing group 1,1-dicyanomethylene-3-indanone, is designed, synthesized, and applied in semitransparent organic solar cells (ST-OSCs). IHIC exhibits strong near-infrared absorption with extinction coefficients of up to 1.6 × 10⁵m⁻¹ cm⁻¹, a narrow optical bandgap of 1.38 eV, and a high electron mobility of 2.4 × 10⁻³ cm² V⁻¹ s⁻¹. The ST-OSCs based on blends of a narrow-bandgap polymer donor PTB7-Th and narrow-bandgap IHIC acceptor exhibit a champion power conversion efficiency of 9.77% with an average visible transmittance of 36% and excellent device stability; this efficiency is much higher than any single-junction and tandem ST-OSCs reported in the literature.

A fused hexacyclic electron acceptor with strong near-infrared absorption and high electron mobility is designed, synthesized, and applied in semitransparent organic solar cells, which exhibit a champion efficiency of 9.77% with an average visible transmittance of 36% and excellent device stability.