All‐Solution‐Processed Cu2ZnSnS4 Solar Cells with Self‐Depleted Na2S Back Contact Modification Layer

The thin‐film photovoltaic material Cu₂ZnSnS₄ (CZTS) has drawn worldwide attention in recent years due to its earth‐abundant, nontoxic element constitution, and remarkable photovoltaic performance. Although state‐of‐the‐art power conversion efficiency is achieved by hydrazine‐based methods, effort to fabricate such devices in a high throughput, environmental‐friendly way is still highlydesired. Here a hydrazine‐free all‐solution‐processed CZTS solar cell with Na₂S self‐depleted back contact modification layer for the first time is demonstrated, using a ball‐milled CZTS as light absorber, low‐temperature solution‐processed ZnO electron‐transport layer as well as silver‐nanowire transparent electrode. The inserting of Na₂S self‐depleted layer is proven to effectively stabilize the CZTS/Mo interface by eliminating a detrimental phase segregation reaction between CZTS and Mo‐coated soda lime glass, thus leading to a better crystallinity of CZTS light absorbing layer, enhanced carrier transportation at CZTS/Mo interface as well as a smaller series resistance. Furthermore, the self‐depletion feature of the Na₂S modification layer also averts hole‐transportation barrier within the devices. The results show the vital importance of interfacial engineering for these CZST devices and the Na₂S interface layer can be extended to other optoelectronic devices using Mo contact.     

Nongeminate Recombination and Charge Transport Limitations in Diketopyrrolopyrrole‐Based Solution‐Processed Small Molecule Solar Cells

Charge transport and nongeminate recombination are investigated in two solution‐processed small molecule bulk heterojunction solar cells consisting of diketopyrrolopyrrole (DPP)‐based donor molecules, mono‐DPP and bis‐DPP, blended with [6,6]‐phenyl‐C71‐butyric acid methyl ester (PCBM). While the bis‐DPP system exhibits a high fill factor (62%) the mono‐DPP system suffers from pronounced voltage dependent losses, which limit both the fill factor (46%) and short circuit current. A method to determine the average charge carrier density, recombination current, and effective carrier lifetime in operating solar cells as a function of applied bias is demonstrated. These results and light intensity measurements of the current‐voltage characteristics indicate that the mono‐DPP system is severely limited by nongeminate recombination losses. Further analysis reveals that the most significant factor leading to the difference in fill factor is the comparatively poor hole transport properties in the mono‐DPP system (2 × 10^?5 cm² V^?1 s^?1 versus 34 × 10^?5 cm² V^?1 s^?1). These results suggest that future design of donor molecules for organic photovoltaics should aim to increase charge carrier mobility thereby enabling faster sweep out of charge carriers before they are lost to nongeminate recombination.     

Polydisperse Spindle‐Shaped ZnO Particles with Their Packing Micropores in the Photoanode for Highly Efficient Quasi‐Solid Dye‐Sensitized Solar Cells

In this paper, a novel hierarchically structured ZnO photoanode for use in quasi‐solid state dye‐sensitized solar cells (DSCs) is presented. The film is composed of polydisperse spindle‐shaped ZnO particles that are prepared through direct precipitation of zinc acetate in aqueous solution. Without additional pore‐forming agents, the microporous structure is well constructed through the packing of polydisperse ZnO particles. In the film, small ZnO particles are able to improve interparticle connectivity and offer a large internal surface area for sufficient dye‐adsorption; on the other hand, particles of larger size can enhance the occurrence of light‐scattering and introduce micropores for the permeation of quasi‐solid state electrolytes. Meanwhile, morphologies, particle size, and specific areas of the products are controlled by altering the reactant concentration and synthetic temperature. Combined with a highly viscous polymer gel electrolyte, a device based on this ZnO photoanode shows high conversion efficiencies, 4.0% and 7.0%, under 100 and 30 mW cm^?2 illumination, respectively. Finally, the unsealed device is demonstrated to remain above 90% of its initial conversion efficiency after 7 days, showing excellent stability.     

High‐Performance Air‐Processed Polymer–Fullerene Bulk Heterojunction Solar Cells

High photovoltaic device performance is demonstrated in ambient‐air‐processed bulk heterojunction solar cells having an active blend layer of organic poly(3‐hexylthiophene) (P3HT): [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM), with power conversion efficiencies as high as 4.1%, which is comparable to state‐of‐the‐art bulk heterojunction devices fabricated in air‐free environments. High‐resolution transmission electron microscopy is combined with detailed analysis of electronic carrier transport in order to quantitatively understand the effects of oxygen exposure and different thermal treatments on electronic conduction through the highly nanostructured active blend network. Improvement in photovoltaic device performance by suitable post‐fabrication thermal processing results from the reduced oxygen charge trap density in the active blend layer and is consistent with a corresponding slight increase in thickness of an ～4 nm aluminum oxide hole‐blocking layer present at the electron‐collecting contact interface.     

Open‐Circuit Voltage and Effective Gap of Organic Solar Cells

The open‐circuit voltage (V_OC) of an organic solar cell is limited by the donor‐acceptor material system. The effective gap E_g^eff between the electron affinity of the acceptor and the ionization potential of the donor is usually regarded as the upper limit for V_OC, which is only reached for T → 0 K. This relation is confirmed for a number of small‐molecule bulk heterojunction p‐i‐n type solar cells by varying the temperature and illumination intensity. With high precision, the low temperature limit of V_OC is identical to E_g^eff. Furthermore, the influence of the hole transport material in a p‐doped hole transport layer and the donor‐acceptor mixing ratio on this limit V₀ is found to be negligible. Varying the active material system, the quantitative relation between V₀ and E_g^eff is found to be identity. A comparison of V₀ in a series of nine different donor‐acceptor material combinations opens a pathway to quantitatively determine the ionization potential of a donor material or the electron affinity of an acceptor material.     

Solution‐Processed Metal Oxide Nanocrystals as Carrier Transport Layers in Organic and Perovskite Solar Cells

There has been rapid progress in solution‐processed organic solar cells (OSCs) and perovskite solar cells (PVSCs) toward low‐cost and high‐throughput photovoltaic technology. Carrier (electron and hole) transport layers (CTLs) play a critical role in boosting their efficiency and long‐time stability. Solution‐processed metal oxide nanocrystals (SMONCs) as a promising CTL candidate, featuring robust process conditions, low‐cost, tunable optoelectronic properties, and intrinsic stability, offer unique advantages for realizing cost‐effective, high‐performance, large‐area, and mechanically flexible photovoltaic devices. In this review, the recent development of SMONC‐based CTLs in OSCs and PVSCs is summarized. This paper starts with the discussion of synthesis approaches of SMONCs. Then, a broad range of SMONC‐based CTLs, including hole transport layers and electron transport layers, are reviewed, in which an emphasis is placed on the improvement of the efficiency and device stability. Finally, for the better understanding of the challenges and opportunities on SMONC‐based CTLs, several strategies and perspectives are outlined.     

Highly Efficient and Bendable Organic Solar Cells with Solution‐Processed Silver Nanowire Electrodes

Highly efficient and bendable organic solar cells (OSCs) are fabricated using solution‐processed silver nanowire (Ag NW) electrodes. The Ag NW films were highly transparent (diffusive transmittance ≈ 95% at a wavelength of 550 nm), highly conductive (sheet resistance ≈ 10 Ω sq^?1), and highly flexible (change in resistance ≈ 1.1 ± 1% at a bending radius of ≈200 μm). Power conversion efficiencies of ≈5.80 and 5.02% were obtained for devices fabricated on Ag NWs/glass and Ag NWs/poly(ethylene terephthalate) (PET), respectively. Moreover, the bendable devices fabricated using the Ag NWs/PET films decrease slightly in their efficiency (to ≈96% of the initial value) even after the devices had been bent 1000 times with a radius of ≈1.5 mm.     

Inverted Organic Solar Cells with Sol–Gel Processed High Work‐Function Vanadium Oxide Hole‐Extraction Layers

For large‐scale and high‐throughput production of organic solar cells (OSCs), liquid processing of the functional layers is desired. We demonstrate inverted bulk‐heterojunction organic solar cells (OSCs) with a sol–gel derived V₂O₅ hole‐extraction‐layer on top of the active organic layer. The V₂O₅ layers are prepared in ambient air using Vanadium(V)‐oxitriisopropoxide as precursor. Without any post‐annealing or plasma treatment, a high work function of the V₂O₅ layers is confirmed by both Kelvin probe analysis and ultraviolet photoelectron spectroscopy (UPS). Using UPS and inverse photoelectron spectroscopy (IPES), we show that the electronic structure of the solution processed V₂O₅ layers is similar to that of thermally evaporated V₂O₅ layers which have been exposed to ambient air. Optimization of the sol gel process leads to inverted OSCs with solution based V₂O₅ layers that show power conversion efficiencies similar to that of control devices with V₂O₅ layers prepared in high‐vacuum.     

Film Morphology of High Efficiency Solution‐Processed Small‐Molecule Solar Cells

Morphological control over the bulk heterojunction (BHJ) microstructure of a high‐efficiency small molecule photovoltaic system is demonstrated using both thermal treatment and solvent additive processing. Single crystal X‐ray diffraction is utilized to understand molecular interactions in the solid state and the BHJ morphology is examined using bright field, high‐resolution, and cross‐section transmission electron microscopy techniques. Controlling the domain size, while maintaining good molecular order within the semiconducting donor material, is found to be crucial in achieving high performance and over 90% internal quantum efficiency exhibited under the optimized conditions.     

A–D–A‐Type Oligothiophenes for Small Molecule Organic Solar Cells: Extending the π‐System by Introduction of Ring‐Locked Double Bonds

A series of novel acceptor–donor–acceptor oligothiophenes terminally substituted with the 1‐(1,1‐dicyanomethylene)‐cyclohex‐2‐ene (DCC) acceptor has been synthesized. Structural, thermal, optoelectronic, and photovoltaic properties of the π‐extended DCCnTs (n = 1–4) are characterized and contrasted to the trends found for the series of parent dicyanovinyl (DCV)‐substituted oligothiophenes DCVnT. The optoelectronic properties reveal the influence of the additional exocyclic, sterically fixed double bonds in trans‐configuration in the novel DCCnT derivatives. A close correspondence for derivatives with equal number of double bonds, that is, DCCnTs and DCV(n + 1)Ts, is identified. Despite having the same energy gap, the energy levels of the frontier orbitals, HOMO and LUMO, for the DCC ‐ derivatives are raised and more destabilized due to the aromatization energy of a thiophene ring versus two exocyclic double bonds indicating improved donor and reduced acceptor strength. DCC‐terthiophenes give good photovoltaic performance as donor materials in vacuum‐processed solar cells (power conversion efficiencies ≤ 4.4%) clearly outperforming all comparable DCV4T derivatives.     

Probing Molecular and Crystalline Orientation in Solution‐Processed Perovskite Solar Cells

The microstructure of solution‐processed organometallic lead halide perovskite thin films prepared by the “gas‐assisted” method is investigated with synchrotron‐based techniques. Using a combination of GIWAXS and NEXAFS spectroscopy the orientational alignment of CH₃NH₃PbI₃ crystallites and CH₃NH₃⁺ cations are separately probed. The GIWAXS results reveal a lack of preferential orientation of CH₃NH₃PbI₃ crystallites in 200–250 nm thick films prepared on both planar TiO₂ and mesoporous TiO₂. Relatively high efficiencies are observed for device based on such films, with 14.3% achieved for planar devices and 12% for mesoporous devices suggesting that highly oriented crystallites are not crucial for good cell performance. Oriented crystallites however are observed in thinner films (≈60 nm) deposited on planar TiO₂ (but not on mesoporous TiO₂) indicating that the formation of oriented crystallites is sensitive to the kinetics of solvent evaporation and the underlying TiO₂ morphology. NEXAFS measurements on all samples found that CH₃NH₃⁺ cations exhibit a random molecular orientation with respect to the substrate. The lack of any NEXAFS dichroism for the thin CH₃NH₃PbI₃ layer deposited on planar TiO₂ in particular indicates the absence of any preferential orientation of CH₃NH₃⁺ cations within the CH₃NH₃PbI₃ unit cell for as‐prepared layers, that is, without any electrical poling.     

Low‐Noise Multispectral Photodetectors Made from All Solution‐Processed Inorganic Semiconductors

Infrared, visible, and multispectral photodetectors are important components for sensing, security and electronics applications. Current fabrication of these devices is based on inorganic materials grown by epitaxial techniques which are not compatible with low‐cost large‐scale processing. Here, air‐stable multispectral solution‐processed inorganic double heterostructure photodetectors, using PbS quantum dots (QDs) as the photoactive layer, colloidal ZnO nanoparticles as the electron transport/hole blocking layer (ETL/HBL), and solution‐derived NiO as the hole transport/electron blocking layer (HTL/EBL) are reported. The resulting device has low dark current density of 20 nA cm^‐2 with a noise equivalent power (NEP) on the order of tens of picowatts across the detection spectra and a specific detectivity (D*) value of 1.2 × 10¹² cm Hz^1/2 W^‐1. These parameters are comparable to commercially available Si, Ge, and InGaAs photodetectors. The devices have a linear dynamic range (LDR) over 65 dB and a bandwidth over 35 kHz, which are sufficient for imaging applications. Finally, these solution‐processed inorganic devices have a long storage lifetime in air, even without encapsulation.     

Solution‐Processed Metallic Nanowire Electrodes as Indium Tin Oxide Replacement for Thin‐Film Solar Cells

Solution processed silver nanowire (Ag NW) films are introduced as transparent electrodes for thin‐film solar cells. Ag NW electrodes were processed by doctor blade‐coating on glass substrates at moderate temperatures (less than 100 °C). The morphological, optical, and electrical characteristics of these electrodes were investigated as a function of processing parameters. For solar‐cell application, Ag NW electrodes with an average transparency of 90% between 450 and 800 nm and a sheet resistivity of ≈10 Ω per square were chosen. The performance of poly(3‐hexylthiophen‐2,5‐diyl):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM) solar cells on Ag NW electrodes was found to match the performance of otherwise identical cells on indium tin oxide. Overall, P3HT:PCBM solar cells with an efficiency of 2.5% on transparent Ag NW electrodes have been realized.     

Band‐Tail Recombination in Hybrid Lead Iodide Perovskite

Traps limit the photovoltaic efficiency and affect the charge transport of optoelectronic devices based on hybrid lead halide perovskites. Understanding the nature and energy scale of these trap states is therefore crucial for the development and optimization of solar cell and laser technology based on these materials. Here, the low‐temperature photoluminescence of formamidinium lead triiodide (HC(NH₂)₂PbI₃) is investigated. A power‐law time dependence in the emission intensity and an additional low‐energy emission peak that exhibits an anomalous relative Stokes shift are observed. Using a rate‐equation model and a Monte Carlo simulation, it is revealed that both phenomena arise from an exponential trap‐density tail with characteristic energy scale of ≈3 meV. Charge‐carrier recombination from sites deep within the tail is found to cause emission with energy downshifted by up to several tens of meV. Hence, such phenomena may in part be responsible for open‐circuit voltage losses commonly observed in these materials. In this high‐quality hybrid perovskite, trap states thus predominantly comprise a continuum of energetic levels (associated with disorder) rather than discrete trap energy levels (associated, e.g., with elemental vacancies). Hybrid perovskites may therefore be viewed as classic semiconductors whose band‐structure picture is moderated by a modest degree of energetic disorder.     

Design of Multilayered Nanostructures and Donor–Acceptor Interfaces in Solution‐Processed Thin‐Film Organic Solar Cells

Multilayered polymer thin‐film solar cells have been fabricated by wet processes such as spin‐coating and layer‐by‐layer deposition. Hole‐ and electron‐transporting layers were prepared by spin‐coating with poly(3,4‐ethylenedioxythiophene) oxidized with poly(4‐styrenesulfonate) (PEDOT:PSS) and fullerene (C₆₀), respectively. The light‐harvesting layer of poly‐(p‐phenylenevinylene) (PPV) was fabricated by layer‐by‐layer deposition of the PPV precursor cation and poly(sodium 4‐styrenesulfonate) (PSS). The layer‐by‐layer technique enables us to control the layer thickness with nanometer precision and select the interfacial material at the donor–acceptor heterojunction. Optimizing the layered nanostructures, we obtained the best‐performance device with a triple‐layered structure of PEDOT:PSS|PPV|C₆₀, where the thickness of the PPV layer was 11 nm, comparable to the diffusion length of the PPV singlet exciton. The external quantum efficiency spectrum was maximum (ca. 20%) around the absorption peak of PPV and the internal quantum efficiency was estimated to be as high as ca. 50% from a saturated photocurrent at a reverse bias of ?3 V. The power conversion efficiency of the triple‐layer solar cell was 0.26% under AM1.5G simulated solar illumination with 100 mW cm^?2 in air.     

HPbI3: A New Precursor Compound for Highly Efficient Solution‐Processed Perovskite Solar Cells

Recently, there have been extensive research efforts on developing high performance organolead halide based perovskite solar cells. While most studies focused on optimizing the deposition processes of the perovskite films, the selection of the precursors has been rather limited to the lead halide/methylammonium (or formamidium) halide combination. In this work, we developed a new precursor, HPbI₃, to replace lead halide. The new precursor enables formation of highly uniform formamidium lead iodide (FAPbI₃) films through a one‐step spin‐coating process. Furthermore, the FAPbI₃ perovskite films exhibit a highly crystalline phase with strong (110) preferred orientation and excellent thermal stability. The planar heterojunction solar cells based on these perovskite films exhibit an average efficiency of 15.4% and champion efficiency of 17.5% under AM 1.5 G illumination. By comparing the morphology and formation process of the perovskite films fabricated from the formamidium iodide (FAI)/HPbI₃, FAI/PbI₂, and FAI/PbI₂ with HI additive precursor combinations, it is shown that the superior property of the HPbI₃ based perovskite films may originate from 1) a slow crystallization process involving exchange of H⁺ and FA⁺ ions in the PbI₆ octahedral framework and 2) elimination of water in the precursor solution state.     

Pore‐Filling of Spiro‐OMeTAD in Solid‐State Dye Sensitized Solar Cells: Quantification,Mechanism, and Consequences for Device Performance

In this paper, the pore filling of spiro‐OMeTAD (2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)9,9′‐spirobifluorene) in mesoporous TiO₂ films is quantified for the first time using XPS depth profiling and UV–Vis absorption spectroscopy. It is shown that spiro‐OMeTAD can penetrate the entire depth of the film, and its concentration is constant throughout the film. We determine that in a 2.5‐µm‐thick film, the volume of the pores is 60–65% filled. The pores become less filled when thicker films are used. Such filling fraction is much higher than the solution concentration because the excess solution on top of the film can act as a reservoir during the spin coating process. Lastly, we demonstrate that by using a lower spin coating speed and higher spiro‐OMeTAD solution concentration, we can increase the filling fraction and consequently the efficiency of the device.     

Interface Engineering for Both Cathode and Anode Enables Low‐Cost Highly Efficient Solution‐Processed CdTe Nanocrystal Solar Cells

Low‐cost solution‐processed CdTe nanocrystal (NC) solar cells always suffer from a high interface energy barrier and unbalanced hole/electron transport as well as anisotropic atom diffusion on the CdTe surface due to the limited amount of hole/electron interface materials or the difficulty in interface processing. In this work, a novel strategy is first adopted with gradient electron transport layer (CdS/CdSe) modification in the cathode and a new crosslinkable hole transport polymer (P‐TPA) implantation in the anode. The carrier recombination at interfaces is greatly decreased and thus the carrier collection is increased. Moreover, the light harvesting is improved both in short and long wavelength regions, making J_sc and V_oc increase simultaneously. A champion solar cell shows a very high power conversion efficiency of 9.2% and an outstanding J_sc of 25.31 mA cm^?2, which are among the highest values for all solution‐processed CdTe NC solar cells with a superstrate structure, and the latter value is even higher than that of traditional thick CdTe thin‐film solar cells (2 µm) via the high temperature close space sublimation method. This work demonstrates that facile surface modifications in both the cathode and anode with stepped extraction and organic–inorganic hybridization are very promising in constructing next‐generation highly efficient NC photovoltaic devices.     

Alternating Copolymers of Cyclopenta[2,1‐b;3,4‐b′]dithiophene and Thieno[3,4‐c]pyrrole‐4,6‐dione for High‐Performance Polymer Solar Cells

A series of alternating copolymers of cyclopenta[2,1‐b;3,4‐b′]dithiophene (CPDT) and thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) have been prepared and characterized for polymer solar cell (PSC) applications. Different alkyl side chains, including butyl (Bu), hexyl (He), octyl (Oc), and 2‐ethylhexyl (EH), are introduced to the TPD unit in order to adjust the packing of the polymer chain in the solid state, while the hexyl side chain on the CPDT unit remains unchanged to simplify discussion. The polymers in this series have a simple main chain structure and can be synthesized easily, have a narrow band gap and a broad light absorption. The different alkyl chains on the TPD unit not only significantly influence the solubility and chain packing, but also fine tune the energy levels of the polymers. The polymers with Oc or EH group have lower HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels, resulting higher open circuit voltages (V_oc) of the PSC devices. Power conversion efficiencies (PCEs) up to 5.5% and 6.4% are obtained from the devices of the Oc substituted polymer (PCPDTTPD‐Oc) with PC₆₁BM and PC₇₁BM, respectively. This side chain effect on the PSC performance is related to the formation of a fine bulk heterojunction structure of polymer and PCBM domains, as observed with atomic force microscopy.