Accelerated Degradation Due to Weakened Adhesion from Li-TFSI Additives in Perovskite Solar Cells

Reliable integration of organometallic halide perovskite in photovoltaic devices is critically limited by its low stability in humid environments. Furthermore, additives to increase the mobility in the hole transport material (HTM) have deliquescence and hygroscopic properties, which attract water molecules and result in accelerated degradation of the perovskite devices. In this study, a double cantilever beam (DCB) test is used to investigate the effects of additives in the HTM layer on the perovskite layer through neatly delaminating the interface between the perovskite and HTM layers. Using the DCB test, the bottom surface of the HTM layers is directly observed, and it is found that the additives are accumulated at the bottom along the thickness (i.e., through-plane direction) of the films. It is also found that the additives significantly decrease the adhesion at the interface between the perovskite and HTM layers by more than 60% through hardening the HTM films. Finally, the adhesion-based degradation mechanism of perovskite devices according to the existence of additives is proposed for humid environments

Development of Highly Crystalline Donor–Acceptor-Type Random Polymers for High Performance Large-Area Organic Solar Cells

We developed donor–acceptor (D–A)-type random polymers based on 3,3′-difluoro-2,2′-bithiophene with various relative amounts of 5,6-difluoro-4,7-bis­(5-bromo-(2-decyl­tetradecyl)­thiophen-2-yl)-2,1,3-benzothiadiazole (2FBT) and 5,6-difluoro-4,7-bis­(5-bromo-(2-octyldodecyl)­thiophen-2-yl)-2-(3,4-dichloro­benzyloxybutyl)-2<i>H</i>-benzo­[<i>d</i>]­[1,2,3]­triazole (DCB-2FBTZ). Introducing small relative amounts of DCB-2FBTZ into the polymer was found to effectively enhance its solar cell performance, resulting in a power conversion efficiency of 9.02%, greater than the 7.29% that resulted from the PFBT-FTh copolymer. Moreover, when the active area of the BHJ film was increased to 1 cm², the solar cell reproducibly showed a high performance, here with an efficiency of 8.01% even when the thickness of the active layer was 313 nm. Our studies revealed that including the DCB-2FBTZ group in the polymer simultaneously improved the solution processability and crystallinity of the polymer. These improvements resulted in the formation of highly homogeneous BHJ films throughout large areas with only minor amounts of defects resulting from overaggregation and hence with appropriate morphologies for effective charge generation and transport

Synthesis and Charge Transport Properties of Conjugated Polymers Incorporating Difluorothiophene as a Building Block

A series of conjugated copolymers, PDPPFT and PNDIFT, were developed using difluoroterthiophene and DPP or NDI as the cobuilding block. We obtained two different molecular weight polymers for each polymer type by changing the conditions for the Stille coupling reaction and studied their optoelectrochemical properties and charge-transport behavior in organic field-effect transistors (OFETs). Both the lower molecular weight polymers, PDPPFT­(L) and PNDIFT­(L), showed better long-range ordered structures in films, whereas the polymers with higher molecular weights were less long-range ordered and showed a more preferential face-on orientation. By virtue of their favorable polymer packing structures, PDPPFT­(L) and PNDIFT­(L) exhibited much higher hole mobilities compared with their higher molecular weight counterparts, PDPPFT­(H) and PNDIFT­(H). By contrast, both PDPPFT and PNDIFT maintained good n-channel properties independent of their molecular weights, thus their long-range ordering in a film. The strong electron-withdrawing fluorine groups are favorable for stabilizing electrons on the polymer chain and would enable the polymer to transport electrons efficiently even in the case of a less-ordered packing structure with an unfavorable face-on orientation

Important Role of Additive in Morphology of Stretchable Electrode for Highly Intrinsically Stable Organic Photovoltaics

Developing intrinsically stretchable organic photovoltaics (IS-OPVs) is crucial for serving as power sources in future portable and wearable electronics. PEDOT:PSS is most commonly used to prepare highly conductive, transparent electrodes with high stretchability. The mechanical properties of PEDOT:PSS films are significantly affected by their morphology, which is primarily determined by the processing additives used. We investigate the effects of two additives, poly(ethylene glycol) (PEG) and (3-glycidyloxypropyl)­trimethoxysilane (GOPS), on the stretchability of the electrode. The PEG additive forms hydrogen bonds with sulfonyl groups of PSS without significant interaction among itself, which releases mechanical stress in the PSS-rich region of the PEDOT:PSS films. On the other hand, the GOPS additive not only forms hydrogen bonds with PSS but also undergoes a chemical reaction to create a cross-linked structure within the film, which effectively enhances the stretchable properties of the PEDOT:PSS film. In addition, the GOPS promotes a more hydrophilic surface compared to PEG, resulting in improved adhesion to the upper layer in IS-OPV devices. This improves the stretchability of IS-OPV devices, as well as their solar cell performance. We demonstrate IS-OPVs that are prepared using GOPS by a non-spin-coating method and these devices exhibit higher performance compared with PEG-based counterparts. Furthermore, the GOPS based IS-OPV shows significantly improved mechanical stability, enabling it to retain 90% of its initial efficiency when subjected to 20% strain

Development of Novel Conjugated Polyelectrolytes as Water-Processable Interlayer Materials for High-Performance Organic Photodiodes

A series of novel conjugated polyelectrolytes composed of two different building blocks with different composition ratios were designed and synthesized for application as a functional layer in high-performance organic photodiodes (OPDs). A homopolymer and two random copolymers were prepared using different molar ratios of dibromo 1,4-bis­(4-sulfonatobutoxy)­benzene (SPh) and dibromo 1,4-bis­(4-tetraethylene glycol)­benzene (EGPh): <b>EG20</b> with SPh:EGPh ratio of 0.8:0.2 and <b>EG40</b> with a ratio of 0.6:0.4. Structural analyses by two-dimensional grazing-incidence X-ray diffraction and near-edge X-ray absorption fine structure spectroscopy studies proved that a higher EGPh content could induce more organized polymer chains with face-on orientation of <b>EG20</b> and <b>EG40</b>. Such an orientation of <b>EG20</b> and <b>EG40</b> along with the ordered crystalline organization yielded effective molecular dipole moments in the thin films when applied as an interlayer between ZnO and an active layer of inverted OPDs. As confirmed by ultraviolet photoelectron spectroscopy, the increase in EG content gradually shifted the workfunction of the ZnO, facilitating the inverted OPD to simultaneously achieve a decrease in dark current and enhancement in photocurrent. The synergetic effects introduced by the newly designed <b>EG20</b> and <b>EG40</b> resulted in significantly improved OPD performances with high specific detectivity up to 2.1 × 10¹³ Jones, 3 dB bandwidth of 72 kHz, and linear dynamic range of 110 dB

Ultrafast Intramolecular Exciton Splitting Dynamics in Isolated Low-Band-Gap Polymers and Their Implications in Photovoltaic Materials Design

Record-setting organic photovoltaic cells with <b>PTB</b> polymers have recently achieved ∼8% power conversion efficiencies (PCE). A subset of these polymers, the <b>PTBF</b> series, has a common conjugated backbone with alternating thieno­[3,4-<i>b</i>]­thiophene and benzodithiophene moieties but differs by the number and position of pendant fluorine atoms attached to the backbone. These electron-withdrawing pendant fluorine atoms fine tune the energetics of the polymers and result in device PCE variations of 2–8%. Using near-IR, ultrafast optical transient absorption (TA) spectroscopy combined with steady-state electrochemical methods we were able to obtain TA signatures not only for the exciton and charge-separated states but also for an intramolecular (“pseudo”) charge-transfer state in isolated <b>PTBF</b> polymers in solution, in the absence of the acceptor phenyl-C₆₁-butyric acid methyl ester (<b>PCBM</b>) molecules. This led to the discovery of branched pathways for intramolecular, ultrafast exciton splitting to populate (a) the charge-separated states or (b) the intramolecular charge-transfer states on the subpicosecond time scale. Depending on the number and position of the fluorine pendant atoms, the charge-separation/transfer kinetics and their branching ratios vary according to the trend for the electron density distribution in favor of the local charge-separation direction. More importantly, a linear correlation is found between the branching ratio of intramolecular charge transfer and the charge separation of hole–electron pairs in isolated polymers versus the device fill factor and PCE. The origin of this correlation and its implications in materials design and device performance are discussed

Highly Efficient Copper–Indium–Selenide Quantum Dot Solar Cells: Suppression of Carrier Recombination by Controlled ZnS Overlayers

Copper–indium–selenide (CISe) quantum dots (QDs) are a promising alternative to the toxic cadmium- and lead-chalcogenide QDs generally used in photovoltaics due to their low toxicity, narrow band gap, and high absorption coefficient. Here, we demonstrate that the photovoltaic performance of CISe QD-sensitized solar cells (QDSCs) can be greatly enhanced simply by optimizing the thickness of ZnS overlayers on the QD-sensitized TiO₂ electrodes. By roughly doubling the thickness of the overlayers compared to the conventional one, conversion efficiency is enhanced by about 40%. Impedance studies reveal that the thick ZnS overlayers do not affect the energetic characteristics of the photoanode, yet enhance the kinetic characteristics, leading to more efficient photovoltaic performance. In particular, both interfacial electron recombination with the electrolyte and nonradiative recombination associated with QDs are significantly reduced. As a result, our best cell yields a conversion efficiency of 8.10% under standard solar illumination, a record high for heavy metal-free QD solar cells to date

Mediating Solar Cell Performance by Controlling the Internal Dipole Change in Organic Photovoltaic Polymers

We report synthesis and characterizations of two novel series of polymers, namely the PBTZ and PBIT series. The PBTZ1 polymer was synthesized as a copolymer of 4,8-bis­(2-butyloctyl)­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (BDT) along with 2,5-bis­(2-ethylhexyl)-3,6-bisthiazol-2-yl-2,5-dihydropyrrolo­[3,4-<i>c</i>]­pyrrole-1,4-dione (TzDPP), while PBTZ2 was a copolymer of TzDPP and 2-(1-butylheptyl)­thieno­[3,4-<i>d</i>]­thiazole (TTz). The PBIT series based on dithienopyrrolobenzothiadiazole (DPBT), and BDT was also synthesized. The PBIT series of polymers showed enhanced ground and excited state dipole moments (μ_g and μ_e) when compared to the previously reported PBB3 polymer, while PBTZ1 showed the largest dipole change (1.52 D) from ground to excited state (Δμ_ge) in respective single polymer units. It was found that the power conversion efficiencies of the polymer series were strongly correlated to Δμ_ge. The results reported demonstrate the utility of the calculated parameter Δμ_ge of single units of the polymers to predict the performance of donor–acceptor copolymers in photovoltaic devices. We rationalize this result based on the large degree of polarization in the excited state, which effectively lowers the Coulomb binding energy of the exciton in the excited state and leads to faster charge separation kinetics, thus facilitating the full separation of electron and hole

High Crystalline Dithienosilole-Cored Small Molecule Semiconductor for Ambipolar Transistor and Nonvolatile Memory

We characterized the electrical properties of a field-effect transistor (FET) and a nonvolatile memory device based on a solution-processable low bandgap small molecule, Si1TDPP-EE-C6. The small molecule consisted of electron-rich thiophene-dithienosilole-thiophene (Si1T) units and electron-deficient diketopyrrolopyrrole (DPP) units. The as-spun Si1TDPP-EE-C6 FET device exhibited ambipolar transport properties with a hole mobility of 7.3 × 10^–5 cm²/(V s) and an electron mobility of 1.6 × 10^–5 cm²/(V s). Thermal annealing at 110 °C led to a significant increase in carrier mobility, with hole and electron mobilities of 3.7 × 10^–3 and 5.1 × 10^–4 cm²/(Vs), respectively. This improvement is strongly correlated with the increased film crystallinity and reduced π–π intermolecular stacking distance upon thermal annealing, revealed by grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM) measurements. In addition, nonvolatile memory devices based on Si1TDPP-EE-C6 were successfully fabricated by incorporating Au nanoparticles (AuNPs) as charge trapping sites at the interface between the silicon oxide (SiO₂) and cross-linked poly­(4-vinylphenol) (<i>c</i>PVP) dielectrics. The device exhibited reliable nonvolatile memory characteristics, including a wide memory window of 98 V, a high on/off-current ratio of 1 × 10³, and good electrical reliability. Overall, we demonstrate that donor–acceptor-type small molecules are a potentially important class of materials for ambipolar FETs and nonvolatile memory applications

Correlation between Polymer Structure and Polymer:Fullerene Blend Morphology and Its Implications for High Performance Polymer Solar Cells

We synthesized four polymers (pT3DPP-HD, pT3DPP-OD, pT2TTDPP-HD, and pT2TTDPP-OD) and characterized their photovoltaic properties as a function of the backbone planarity, alkyl side chain length, and film morphology. The polymers were donor–acceptor type low-band-gap (1.2–1.3 eV) polymers employing terthiophene (T3) or thiophene–thieno­[3,2-<i>b</i>]­thiophene–thiophene (T2TT) as the donor and 2,5-bis­(2-hexyldecyl)­pyrrolo­[3,4-<i>c</i>]­pyrrole-1,4-(2<i>H</i>,5<i>H</i>)-dione (DPP-HD) or 2,5-bis­(2-octyldodecyl)­pyrrolo­[3,4-<i>c</i>]­pyrrole-1,4-(2<i>H</i>,5<i>H</i>)-dione (DPP-OD) as the acceptor. The T2TT moiety in the polymer backbone is more planar than the T3; the OD moiety as the alkyl side chain ensured a higher solubility than the HD moiety. Polymer solar cells (PSCs) were fabricated, and their properties were characterized. The photoactive layer consisted of one of the four polymers and one of the fullerene derivatives (PC₇₀BM or PC₆₀BM). For a given fullerene derivative, the PCEs prepared with each of the four polymers were ordered according to pT3DPP-OD, pT2TTDPP-HD, pT3DPP-HD, and pT2TTDPP-OD. Studies on the morphologies of the polymer:fullerene layers revealed that the pT3DPP-OD:PC₇₀BM blend exhibited an optimal degree of phase separation between the polymer and the fullerene, while retaining a high degree of interconnectivity, thereby yielding the highest PCE measured in this series. By contrast, the pT2TTDPP-OD:fullerene yielded the lowest PCE because of too high crystalline fibrous polymer domains. In conclusion, we demonstrate that minute variations in the polymer chemical structure strongly affects both (i) the nanoscale miscibility between the polymers and fullerenes and (ii) the interconnectivity of the polymer chains, and these properties are tightly correlated with the solar cell performance