Chen Weijie

High photon energy losses limit the open-circuit voltage (V_OC) and power conversion efficiency of organic solar cells (OSCs). In this work, an optimization route is presented which increases the V_OC by reducing the interfacial area between donor (D) and acceptor (A). This optimization route concerns a cascade device architecture in which the introduction of discontinuous interlayers between alpha-sexithiophene (α-6T) (D) and chloroboron subnaphthalocyanine (SubNc) (A) increases the V_OC of an α-6T/SubNc/SubPc fullerene-free cascade OSC from 0.98 V to 1.16 V. This increase of 0.18 V is attributed solely to the suppression of nonradiative recombination at the D–A interface. By accurately measuring the optical gap (E_opt) and the energy of the charge-transfer state (E_CT) of the studied OSC, a detailed analysis of the overall voltage losses is performed. E_opt – qV_OC losses of 0.58 eV, which are among the lowest observed for OSCs, are obtained. Most importantly, for the V_OC-optimized devices, the low-energy (700 nm) external quantum efficiency (EQE) peak remains high at 79%, despite a minimal driving force for charge separation of less than 10 meV. This work shows that low-voltage losses can be combined with a high EQE in organic photovoltaic devices.

The insertion of a thin interlayer at the D/A interface of a cascade organic solar cell leads to a reduction of the voltage losses by 180 mV, while maintaining peak external quantum efficiencies approaching 80%. A detailed analysis of the organic multilayer device with exceptionally low losses reveals a minimal driving force for charge separation and suppressed nonradiative recombination.

Significant efforts have lead to demonstrations of nonfullerene solar cells (NFSCs) with record power conversion efficiency up to ≈13% for polymer:small molecule blends and ≈9% for all-polymer blends. However, the control of morphology in NFSCs based on polymer blends is very challenging and a key obstacle to pushing this technology to eventual commercialization. The relations between phases at various length scales and photovoltaic parameters of all-polymer bulk-heterojunctions remain poorly understood and seldom explored. Here, precise control over a multilength scale morphology and photovoltaic performance are demonstrated by simply altering the concentration of a green solvent additive used in blade-coated films. Resonant soft X-ray scattering is used to elucidate the multiphasic morphology of these printed all-polymeric films and complements with the use of grazing incidence wide-angle X-ray scattering and in situ spectroscopic ellipsometry characterizations to correlate the morphology parameters at different length scales to the device performance metrics. Benefiting from the highest relative volume fraction of small domains, additive-free solar cells show the best device performance, strengthening the advantage of single benign solvent approach. This study also highlights the importance of high volume fraction of smallest domains in printed NFSCs and organic solar cells in general.

Precise control over the multilength scale morphology of printed all-polymer photovoltaic films by altering the concentration of a green additive is demonstrated by means of resonant soft X-ray scattering, in situ spectroscopic ellipsometry, and complementary methods. Additive-free nonfullerene devices show the best device performance due to the highest relative volume fraction of small domains and minimized length scale of large domains.

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.

Perovskite solar cells typically use TiO₂ as charge extracting materials, which reduce the photostability of perovskite solar cells under illumination (including ultraviolet light). Simultaneously realizing the high efficiency and photostability, it is demonstrated that the rationally designed iron(III) oxide nanoisland electrodes consisting of discrete nanoislands in situ growth on the compact underlayer can be used as compatible and excellent electron extraction materials for perovskite solar cells. The uniquely designed iron(III) oxide electron extraction layer satisfies the good light transmittance and sufficient electron extraction ability, resulting in a promising power conversion efficiency of 18.2%. Most importantly, perovskite solar cells fabricated with iron(III) oxide show a significantly improved UV light and long-term operation stabilities compared with the widely used TiO₂-based electron extraction material, owing to the low photocatalytic activity of iron(III) oxide. This study highlights the potential of incorporating new charge extraction materials in achieving photostable and high efficiency perovskite photovoltaic devices.

A photostable and efficient perovskite solar cell is presented, employing the rationally designed iron(III) oxide nanoarchitecture consisting of discrete nanoislands in situ growth on the compact underlayer as electron extraction layer. Perovskite solar cells fabricated with iron(III) oxide nanoislands exhibit high power conversion efficiency (over 18%) and promising ultraviolet light and long-term operational stabilities.

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%).

Bismuth-based compounds have recently gained increasing attention as potentially nontoxic and defect-tolerant solar absorbers. However, many of the new materials recently investigated show limited photovoltaic performance. Herein, one such compound is explored in detail through theory and experiment: bismuth oxyiodide (BiOI). BiOI thin films are grown by chemical vapor transport and found to maintain the same tetragonal phase in ambient air for at least 197 d. The computations suggest BiOI to be tolerant to antisite and vacancy defects. All-inorganic solar cells (ITO|NiO_x|BiOI|ZnO|Al) with negligible hysteresis and up to 80% external quantum efficiency under select monochromatic excitation are demonstrated. The short-circuit current densities and power conversion efficiencies under AM 1.5G illumination are nearly double those of previously reported BiOI solar cells, as well as other bismuth halide and chalcohalide photovoltaics recently explored by many groups. Through a detailed loss analysis using optical characterization, photoemission spectroscopy, and device modeling, direction for future improvements in efficiency is provided. This work demonstrates that BiOI, previously considered to be a poor photocatalyst, is promising for photovoltaics.

Bismuth oxyiodide (BiOI) is demonstrated to be defect-tolerant, with the bulk phase unchanged after 197 d in ambient air. In solar cells, up to 80% external quantum efficiency is achieved. The short-circuit current densities and power conversion efficiencies are nearly double previous reports of photovoltaics based on BiOI, as well as other recently explored bismuth halides and chalcohalides.

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.

Large area flexible electronics rely on organic or hybrid materials prone to degradation limiting the device lifetime. For many years, photo-oxidation has been thought to be one of the major degradation pathways. However, intense illumination may lead to a burn-in or a rapid decrease in performance for devices completely isolated from corrosive elements as oxygen or moisture. The experimental studies which are presented in here indicate that a plausible triggering for the burn-in is a spin flip after a UV photon absorption leading to the accumulation of electrostatic potential energy that initiates a rapid destruction of the nanomorpholgy in the fullerene phase of a polymer cell. To circumvent this and achieve highly stable and efficient devices, a robust nanocrystalline ordering is induced in the PCBM phase prior to UV illumination. In that event, PTB7-Th:PC₇₁BM cells are shown to exhibit T₈₀ lifetimes larger than 1.6 years under a continuous UV-filtered 1-sun illumination, equivalent to 7 years for sunlight harvesting at optimal orientation and 10 years for vertical applications.

In polymer cells, a spin flip at the donor/acceptor interface after the absorption of high energy photons leads to the accumulation of electrostatic potential energy initiating a rapid destruction of the fullerene nanomorpholgy. By inducing a robust nanocrystalline ordering in the fullerene phase, PTB7-Th cells with high efficiency (≈9%) and long lifetime (>7 and >10 years for optimal and vertical orientations, respectively) are fabricated.

Solution-processable small molecule (SM) donors are promising alternatives to their polymer counterparts in bulk-heterojunction (BHJ) solar cells. While SM donors with favorable spectral absorption, self-assembly patterns, optimum thin-film morphologies, and high carrier mobilities in optimized donor–acceptor blends are required to further BHJ device efficiencies, material structure governs each one of those attributes. As a result, the rational design of SM donors with gradually improved BHJ solar cell efficiencies must concurrently address: (i) bandgap tuning and optimization of spectral absorption (inherent to the SM main chain) and (ii) pendant-group substitution promoting structural order and mediating morphological effects. In this paper, the rational pendant-group substitution in benzo[1,2-b:4,5-b′]dithiophene–6,7-difluoroquinoxaline SMs is shown to be an effective approach to narrowing the optical gap (E_opt) of the SM donors (SM1 and SM2), without altering their propensity to order and form favorable thin-film BHJ morphologies with PC₇₁BM. Systematic device examinations show that power conversion efficiencies >8% and open-circuit voltages (V_OC) nearing 1 V can be achieved with the narrow-gap SM donor analog (SM2, E_opt = 1.6 eV) and that charge transport in optimized BHJ solar cells proceeds with minimal, nearly trap-free recombination. Detailed device simulations, light intensity dependence, and transient photocurrent analyses emphasize how carrier recombination impacts BHJ device performance upon optimization of active layer thickness and morphology.

Rational pendant-group substitutions in benzo[1,2-b:4,5-b′]dithiophene-6,7-difluoroquin-oxaline small molecule donor analogs yield power conversion efficiencies >8% and high open-circuit voltages nearing 1 V in bulk heterojunction (BHJ) solar cells with the fullerene acceptor PC₇₁BM. Charge transport in optimized BHJ solar cells proceeds with minimal, nearly trap-free recombination.

2D halide perovskite materials have shown great advantages in terms of stability when applied in a photovoltaic device. However, the impediment of charge transport within the layered structure drags down the device performance. Here for the first time, a 3D–2D (MAPbI₃-PEA₂Pb₂I₄) graded perovskite interface is demonstrated with synergistic advantages. In addition to the significantly improved ambient stability, this graded combination modifies the interface energy level in such a way that reduces interface charge recombination, leading to an ultrahigh V_oc at 1.17 V, a record for NiO-based p-i-n photovoltaic devices. Moreover, benefiting from the graded structure induced continuously upshifts energy level, the photovoltaic device attains a high J_sc of 21.80 mA cm⁻² and a high fill factor of 0.78, resulting in an overall power conversion efficiency (PCE) of 19.89%. More importantly, it is showed that such a graded interface structure also suppresses ion migration in the device, accounting for its significantly enhanced thermal stability.

A designer cross-dimensional perovskite–perovskite (3D–2D) interface upshifts the energy level toward the surface, pronouncedly reduces the charge recombination in perovskite photovoltaics, leading to a record high V_OC at 1.17 V. Strongly enhanced ambient and thermal stability is also demonstrated.

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.

A high level of automation is desirable to facilitate the lab-to-fab process transfer of the emerging perovskite-based solar technology. Here, an automated aerosol-jet printing technique is introduced for precisely controlling the thin-film perovskite growth in a planar heterojunction p–i–n solar cell device structure. The roles of some of the user defined parameters from a computer-aided design file are studied for the reproducible fabrication of pure CH₃NH₃PbI₃ thin films under near ambient conditions. Preliminary power conversion efficiencies up to 15.4% are achieved when such films are incorporated in a poly(3,4-ethylenedioxythiophene):polystyrene sulfonate-perovskite-phenyl-C71-butyric acid methyl ester type device format. It is further shown that the deposition of atomized materials in the form of a gaseous mist helps to form a highly uniform and PbI₂ residue-free CH₃NH₃PbI₃ film and offers advantages over the conventional two-step solution approach by avoiding the detrimental solid–liquid interface induced perovskite crystallization. Ultimately, by integrating full 3D motion control, the fabrication of perovskite layers directly on a 3D curved surface becomes possible. This work suggests that 3D automation with aerosol-jet printing, once fully optimized, could form a universal platform for the lab-to-fab process transfer of solution-based perovskite photovoltaics and steer development of new design strategies for numerous embedded structural power applications.

Aerosol-jet printing is applied to mitigate defects during hybrid perovskite thin film growth in an all-low temperature, solution-processing scheme. The high level of automation in the printing process also enables direct write of perovskite semiconductors on a curved surface for photovoltaic device applications. This method could find use in fabricating embedded power components.

Large-scale high-quality perovskite thin films are crucial to produce high-performance perovskite solar cells. However, for perovskite films fabricated by solvent-rich processes, film uniformity can be prevented by convection during thermal evaporation of the solvent. Here, a scalable low-temperature soft-cover deposition (LT-SCD) method is presented, where the thermal convection-induced defects in perovskite films are eliminated through a strategy of surface tension relaxation. Compact, homogeneous, and convection-induced-defects-free perovskite films are obtained on an area of 12 cm², which enables a power conversion efficiency (PCE) of 15.5% on a solar cell with an area of 5 cm². This is the highest efficiency at this large cell area. A PCE of 15.3% is also obtained on a flexible perovskite solar cell deposited on the polyethylene terephthalate substrate owing to the advantage of presented low-temperature processing. Hence, the present LT-SCD technology provides a new non-spin-coating route to the deposition of large-area uniform perovskite films for both rigid and flexible perovskite devices.

For solvent-rich scalable processes, solvents in liquid films are thermally evaporated to form solid films. Due to the temperature difference during heating, a surface-tension difference in the perovskite precursor is established and drives convection to form film defects in cellular patterns. The cellular defects can be eliminated via soft-cover deposition through a strategy of surface tension relaxation.

Magnetic data storage and magnetically actuated devices are conventionally controlled by magnetic fields generated using electric currents. This involves significant power dissipation by Joule heating effect. To optimize energy efficiency, manipulation of magnetic information with lower magnetic fields (i.e., lower electric currents) is desirable. This can be accomplished by reducing the coercivity of the actuated material. Here, a drastic reduction of coercivity is observed at room temperature in thick (≈600 nm), nanoporous, electrodeposited Cu–Ni films by simply subjecting them to the action of an electric field. The effect is due to voltage-induced changes in the magnetic anisotropy. The large surface-area-to-volume ratio and the ultranarrow pore walls of the system allow the whole film, and not only the topmost surface, to effectively contribute to the observed magnetoelectric effect. This waives the stringent “ultrathin-film requirement” from previous studies, where small voltage-driven coercivity variations were reported. This observation expands the already wide range of applications of nanoporous materials (hitherto in areas like energy storage or catalysis) and it opens new paradigms in the fields of spintronics, computation, and magnetic actuation in general.

A novel effect in nanoporous magnetic films is demonstrated: the possibility to drastically reduce their coercivity under the action of an electric field, by simply applying continuous voltage. The reduction of coercivity with voltage implies that lower currents are needed to switch the magnetization of the system, thus considerably reducing heat dissipation and enhancing energy efficiency during magnetic actuation.

Recent technological advances in conventional planar and microstructured solar cell architectures have significantly boosted the efficiencies of these devices near the corresponding theoretical values. Nanomaterials and nanostructures have promising potential to push the theoretical limits of solar cell efficiency even higher using the intrinsic advantages associated with these materials, including efficient photon management, rapid charge transfer, and short charge collection distances. However, at present the efficiency of nanostructured solar cells remains lower than that of conventional solar devices due to the accompanying losses associated with the employment of nanomaterials. The concurrent design of both optical and electrical components will presumably be an imperative route toward breaking the present-day limit of nanostructured solar cells. This review summarizes the losses in traditional solar cells, and then discusses recent advances in applications of nanotechnology to solar devices from both optical and electrical perspectives. Finally, a rule for nanostructured solar cells by concurrently engineering the optical and electrical design is devised. Following these guidelines should allow for exceeding the theoretical limit of solar cell efficiency soon.

Nanostructures produce unique optical and electronic properties, which have the potential to meet the goals of third-generation photovoltaic devices. However, most nanostructures bring accompanying optical or electrical losses to solar cells. Here, it is postulated that the concurrent design of both optical and electrical components will be an imperative route toward breaking the present-day limit of nanostructured solar cells.

In article number 1606656, Yiwang Chen, Thomas Riedl, and co-workers report perovskite solar cells based on an ITO-free bottom electrode. The impermeable SnO_x electron-extraction layer is a hole blocker and at the same time it protects the ultrathin silver bottom electrode against corrosion due to the halide-containing precursors of the perovskite.

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.

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.

Organic–inorganic hybrid perovskite multijunction solar cells have immense potential to realize power conversion efficiencies (PCEs) beyond the Shockley–Queisser limit of single-junction solar cells; however, they are limited by large nonideal photovoltage loss (V _oc,loss) in small- and large-bandgap subcells. Here, an integrated approach is utilized to improve the V _oc of subcells with optimized bandgaps and fabricate perovskite–perovskite tandem solar cells with small V _oc,loss. A fullerene variant, Indene-C₆₀ bis-adduct, is used to achieve optimized interfacial contact in a small-bandgap (≈1.2 eV) subcell, which facilitates higher quasi-Fermi level splitting, reduces nonradiative recombination, alleviates hysteresis instabilities, and improves V _oc to 0.84 V. Compositional engineering of large-bandgap (≈1.8 eV) perovskite is employed to realize a subcell with a transparent top electrode and photostabilized V _oc of 1.22 V. The resultant monolithic perovskite–perovskite tandem solar cell shows a high V _oc of 1.98 V (approaching 80% of the theoretical limit) and a stabilized PCE of 18.5%. The significantly minimized nonideal V _oc,loss is better than state-of-the-art silicon–perovskite tandem solar cells, which highlights the prospects of using perovskite–perovskite tandems for solar-energy generation. It also unlocks opportunities for solar water splitting using hybrid perovskites with solar-to-hydrogen efficiencies beyond 15%.

High open-circuit voltage, V _oc (1.98 V) and power conversion efficiency, PCE (18.5%) is realized in an ideal bandgap-matched two-terminal perovskite–perovskite tandem solar cell via an integrated approach. A fullerene variant, Indene-C₆₀ bis-adduct is used to achieve optimized interfacial contact and alleviate hysteresis instabilities in the small-bandgap subcell. Compositional engineering is employed to realize more highly photostabilized V _oc in the large-bandgap subcell.