Bulk Heterojunction Perovskite Solar Cells Incorporated with Zn₂SnO₄ Nanoparticles as the Electron Acceptors

Perovskite solar cells were developed very fast in the past decade, but hybrid perovskite materials with unbalanced charge carrier diffusion lengths were not fully addressed by either conventional or planar heterojunction device structures. In this study, high-performance perovskite solar cells with bulk heterojunction device structures where CH3NH3PbI2.55Br0.45 is blended with an n-type high-electron-mobility Zn2SnO4 nanoparticle as the photoactive layer are reported. Systematic studies indicate that the CH3NH3PbI2.55Br0.45:Zn2SnO4 bulk heterojunction thin film possesses enhanced and balanced charge carrier mobilities, superior film morphology with enlarged crystal sizes, and suppressed trap density. Photoluminescence and time-resolved photoluminescence studies further demonstrate that there is an efficient photoinduced charge carrier transfer between CH3NH3PbI2.55Br0.45 and Zn2SnO4 nanoparticles. Thus, bulk heterojunction perovskite solar cells by the CH3NH3PbI2.55Br0.45:Zn2SnO4 thin film exhibit over 21.07% power conversion efficiency, which is more than 12% enhancement as compared to that (18.74%) observed from planar heterojunction perovskite solar cells by the pristine CH3NH3PbI2.55Br0.45 thin film. Moreover, bulk heterojunction perovskite solar cells possess significantly suppressed photocurrent hysteresis, dramatically enhanced device stability, and reproducibility. All these results demonstrate that high-performance perovskite solar cells can be realized through bulk heterojunction device structures

Efficient Perovskite Solar Cells through Suppressed Nonradiative Charge Carrier Recombination by a Processing Additive

It has been reported that nonradiative charge carrier recombination in hybrid perovskite materials restricts the device performance of perovskite solar cells. In this study, we report efficient perovskite solar cells through suppressed nonradiative charge carrier recombination by a processing additive, aminopropanoic acid. It is found that aminopropanoic acid not only modulates the crystal growth processes but also minimizes the defects of CH3NH3PbI3 thin films. Moreover, the CH3NH3PbI3 thin films processed with the addition of aminopropanoic acid exhibit both enhanced photoluminescence and electroluminescence and elongated charge carrier lifetime, indicating that nonradiative charge carrier recombination within the CH3NH3PbI3 thin films is drastically suppressed. As a result, perovskite solar cells fabricated using the CH3NH3PbI3 thin films processed with the addition of aminopropanoic acid exhibit approximately 15% enhanced efficiency as compared with those made with pristine CH3NH3PbI3 thin films. All of these results demonstrate that our findings provide a facile way to improve the efficiency of perovskite solar cells

Modifying the Chemical Structure of a Porphyrin Small Molecule with Benzothiophene Groups for the Reproducible Fabrication of High Performance Solar Cells

A porphyrin-based molecule DPPEZnP-BzTBO with bulky benzothiophene groups was designed and synthesized as an electron donor material for bulk heterojunction (BHJ) solar cells. The optimized devices under thermal annealing (TA) and then chloroform solvent vapor anneanling (SVA) for 80 s exhibited an outstanding power conversion efficiencie (PCE) of 9.08%. Contrasted with the smaller thienyl substituted analogues we reported previously, DPPEZnP-BzTBO-based BHJ solar cells exhibited a higher open circuit voltage due to the lower highest occupied molecular orbital energy level. The TA post-treatment of the active layers induced the formation of more crystallized components, and the subsequent SVA provided a driving force for the domain growth, resulting in more obvious phase segregation between the donor and the acceptor in nanoscale. Furthermore, the PCEs kept above 95% upon the further SVA treatment within the time range of 60 to 95 s probably because the bulky benzothiophene groups retard the too quick change of crystallinity, providing a wide processing window for the reproducible device fabrication

Synthesis of Anthracene-Based Donor–Acceptor Copolymers with a Thermally Removable Group for Polymer Solar Cells

A highly soluble anthracene cyclic adduct with a thermally cleavable substituent was synthesized, and it was used as a donor unit in a series of donor–acceptor type conjugated copolymers with improved processability. The removable group was eliminated under elevated temperature through retro Diels–Alder reaction, which offered the corresponding copolymers with better planarity and rigidity. Thermogravimetric analysis (TGA), FT-IR, and UV–vis spectroscopy were carried out to study the thermal cleavage process. Uniform films were easily formed from these precursor copolymers due to their good solution processabilty. Polymer solar cells were successfully fabricated through applying thermal annealing treatment on the blend films that were spin-coated from solutions of the precursor copolymers blended with fullerene derivatives. The best polymer solar cell device with a power conversion efficiency (PCE) of 2.15% was achieved based on copolymer PCOAEHDPP

Efficient Polymer Solar Cells by Lithium Sulfonated Polystyrene as a Charge Transport Interfacial Layer

In this paper, we report the highly efficient bulk heterojunction (BHJ) polymer solar cells (PSCs) with an inverted device structure via utilizing an ultrathin layer of lithium sulfonated polystyrene (LiSPS) ionomer to reengineer the surface of the solution-processed zinc oxide (ZnO) electron extraction layer (EEL). The unique lithium-ionic conductive LiSPS contributes to enhanced electrical conductivity of the ZnO/LiSPS EEL, which not only facilitates charge extraction from the BHJ active layer but also minimizes the energy loss within the charge transport processes. In addition, the organic–inorganic LiSPS ionomer well circumvents the coherence issue of the organic BHJ photoactive layer on the ZnO EEL. Consequently, the enhanced charge transport and the lowered internal resistance between the BHJ photoactive layer and the ZnO/LiSPS EEL give rise to a dramatically reduced dark saturation current density and significantly minimized charge carrier recombination. As a result, the inverted BHJ PSCs with the ZnO/LiSPS EEL exhibit an approximatively 25% increase in power conversion efficiency. These results indicate our strategy provides an easy, but effective, approach to reach high performance inverted PSCs

Fine Emission Tuning from Near-Ultraviolet to Saturated Blue with Rationally Designed Carbene-Based [3 + 2 + 1] Iridium(III) Complexes

We designed and synthesized a new class of six phosphorescent [3 + 2 + 1] iridium­(III) complexes [(pbib)­Ir­(C^C)­CN] bearing a tridentate 1,3-bis­(1-butylimidazolin-2-ylidene) phenyl N-heterocyclic carbene (NHC)-based pincer ligand (pbib), bidentate imidazole-based NHC ligands (C^C), and a monodentate cyano group and investigated their photophysical, electrochemical, and thermal stabilities and electroluminescent properties. The extended π-conjugation of the imidazole-based C^C ligand is found to be the key to fine-tune the emission energies from ultraviolet blue (λ = 378 nm) to saturated blue (λ = 482 nm), as shown by electrochemical and photophysical studies, which is also revealed by the density functional theory (DFT) and time-dependent DFT calculations. Vacuum-deposited organic light-emitting diode devices have been fabricated with these newly synthesized emitters and exhibited the best external quantum efficiency of 6.4% and Commission International de L’Éclairage (CIE) coordinates of (0.163, 0.096), where the CIE y is very similar to the National Television System Committee standard blue CIE (x, y) coordinates of (0.149, 0.085). These results indicate that the novel [3 + 2 + 1] coordinated iridium­(III) complexes [(pbib)­Ir­(C^C)­CN], having a saturated blue emission, not only could alleviate the photodegradation of the emitters when compared to [(pbib)­Ir­(pmi)­CN] but also provide new design strategies of saturated-blue-emitting iridium­(III) complexes

Minimizing Voltage Loss in Efficient All-Inorganic CsPbI₂Br Perovskite Solar Cells through Energy Level Alignment

All-inorganic CsPbI2Br, prized for its strong stability against thermal aging and light soaking, has attracted intensive attention. However, a large energy loss results from the serious energy level offset of 1.05 eV between CsPbI2Br and Spiro-MeOTAD, hindering the further efficiency improvement of perovskite solar cells. To address this issue, a moderate energy level (CsPbI2Br)1–x(CsPbI3)x layer has been introduced at the interface between CsPbI2Br and Spiro-MeOTAD to form a graded energy level alignment, the interpolation of which has offered the energy level gradient for reducing the resistance of hole transport. Correspondingly, the energy level tailoring has minimized the energy loss, and a remarkable VOC improved from 1.12 to 1.32 V, which is one of the highest values for CsPbI2Br-based solar cells. A relatively good thermal stability has also been validated. These good performances indicate that setting an intermediate energy level alignment will be a potential strategy for idealized device architecture to minimize energy loss