Improving the performance of perovskite solar cells with glycerol-doped PEDOT:PSS buffer layer*

Abstract: Organometal halide perovskite solar cells have been constructed using soluble tetra-n-butyl-copper phthalocyanine as hole transporting material. Devices were constructed and characterized under ambient conditions of 50–60% ambient humidity. Soluble copper phthalocyanine gave a modest PCE of 7.3% but when a buffer layer of either Al2O3 or graphene oxide was introduced between the perovskite and the hole transporting layer the cell efficiency extensively increased and reached 14.4% in the presence of graphene oxide. Corresponding data obtained by employing the standard spiro-OMeTAD as hole transporter gave equivalent performance. Combination then of tetra-n-butyl-copper phthalocyanine with graphene oxide offers a very good alternative of simpler and stable materials for perovskite solar cell construction. The presently recorded data highlight the role of the buffer layer, especially graphene oxide, as the material which blocks shunt paths and facilitates hole transfer between the perovskite and the hole transporting layer.

Abstract: The current work reports the lithium (Li) doping of a low-temperature processed zinc oxide (ZnO) electron transport layer (ETL) for highly efficient, triple-cation-based MA0.57FA0.38Rb0.05PbI3 (MA: methylammonium, FA: formamidinium, Rb: rubidium) perovskite solar cells (PSCs). Lithium intercalation in the host ZnO lattice structure is dominated by interstitial doping phenomena, which passivates the intrinsic defects in ZnO film. In addition, interstitial Li doping also downshifts the Fermi energy position of Li-doped ETL by 30 meV, which contributes to the reduction of the electron injection barrier from the photoactive perovskite layer. Compared to the pristine ZnO, the power conversion efficiency (PCE) of the PSCs incorporating lithium-doped ZnO (Li-doped) is raised from 14.07 to 16.14%. The superior performance is attributed to the reduced current leakage, enhanced charge extraction characteristics, and mitigated trap-assisted recombination phenomena in Li-doped devices, thoroughly investigated by mean...

Abstract: The presented work demonstrates the development of highly stable low temperature processed Cesium compound incorporated ZnO electron transport layer (ETL) for perovskite solar cells (PSCs). Cesium compounds such as CA (cesium acetate) and CC (cesium carbonate) modified ETLs are employed for fabricating highly efficient (PCE: ~ 16.5%) mixed organic cation based MA0.6FA0.4PbI3 PSCs via restricted volume solvent annealing (RVSA) method. Here, CA ETL demonstrates a 50 meV upshift in Fermi level position with respect to CC ETL, contributing to higher n-type conductivity and lower electron injection barrier at the interface. Furthermore, CA ETL also exhibits profound influence on the perovskite microstructure leading to larger grain size and uniform distribution. Cesium acetate incorporated devices exhibit about 82% higher PCE compared to conventional CC devices. In addition to higher photovoltaic performance, CA devices exhibit mitigated photo-current hysteresis phenomena compared to CC devices, owing to suppressed electrode polarization phenomena. Besides, the stability of the CA devices are 400% higher than the conventional CC devices, retaining almost 90% of its initial PCE even after a month-long (30 days) systematic degradation study. The mechanism behind superior performance and stability is investigated and discussed comprehensively.

Abstract: Perovskite solar cells have been constructed under the standard procedure by employing soluble tetratriphenylamine-substituted Zn phthalocyanine as hole transporting material. Solution processed device construction was carried out under ambient conditions of 50–60% ambient humidity. Triphenylamine substitution played the double role of imparting solubility to the core metal phthalocyanine as well as to introduce electron-rich ligands, which could enhance the role of Zn phthalocyanine as hole transporter. Indeed, the obtained material was functional. The present data highlight tetratriphenylamine-substituted Zn phthalocyanine as hole transporting material but also highlight the importance of the presence of a buffer layer between the perovskite layer and the hole-transporting layer. Thus the efficiency of the cells was 9.0% in the absence but increased to 13.65% in the presence of Al2O3 buffer layer.

Abstract: Here we report, a controlled primary nucleation aided restricted volume solvent annealing method [NR method, NR stands for nucleation assisted restricted volume solvent annealing (RVSA)] along with mixed organic cation based perovskite film for highly efficient (highest PCE: 16.78%), thermally stable perovskite solar cells (PSCs) with full temperature compatibility for flexible substrates. To the best of our knowledge, this is the highest efficiency ever reported with PSCs incorporating low temperature processed ZnO as electron transport layer (ETL). Unlike the conventional RVSA method [CR method, CR refers to conventional restricted volume solvent annealing (RVSA)], the reported NR method raises the degree of supersaturation of the precursor solution due to high solvent evaporation rate during the crucial primary nucleation phase, that ensures smooth and uniform perovskite grain distribution during the rapid crystal growth phase. NR method also provides better perovskite crystallinity during the annealing induced secondary crystallization phase, as unlike CR, NR perovskite does not contain high amount of residual solvent at this stage of perovskite grain growth. The photovoltaic performance, hysteretic behaviour and device degradation phenomena of NR PSCs have been compared with CR PSCs. Enhanced power conversion efficiency (PCE) attained with NR method has been explained with elaborate surface morphology and topography study, along with charge transport analysis with electrochemical impedance spectroscopy and Mott-Schottky analysis. Substantially mitigated photo-current hysteresis phenomenon with NR method has been elucidated in terms of inherent electrode polarization process. Adding to the merits, NR PSCs also demonstrate over three times higher device stability compared to CR PSCs, which has been investigated with frequency-dependent capacitive spectra and Mott-Schottky characterization of the aged devices.

Abstract: All-solid-state donor/acceptor planar-heterojunction (PHJ) hybrid solar cells are constructed and their excellent performance measured. The deposition of a thin C60 fullerene or fullerene-derivative (acceptor) layer in vacuum on a CH3 NH3 PbI3 perovskite (donor) layer creates a hybrid PHJ that displays the photovoltaic effect. Such heterojunctions are shown to be suitable for the development of newly structured, hybrid, efficient solar cells.

Abstract: Hysteresis-less inverted ITO/PEDOT:PSS/CH3NH3PbI3 (MAPbI3)/PCBM/Au planar hybrid solar cells with 18.1% average power conversion efficiency irrespective of the scan rate were fabricated by depositing dense pinhole-free MAPbI3 perovskite on a PEDOT:PSS/ITO substrate via a single-step spin-coating of solubility controlled MAPbI3 solution. The conductivities of PEDOT:PSS, PCBM, poly(triaryl amine) (PTAA):tert-butylpyrridne (tBP) + Li-bis(trifluoromethanesulfonyl)imide (Li–TFSI), MAPbI3, and TiO2 were 0.014, 0.016, 0.034, 0.015, and 0.00006 mS cm−1, respectively. The average PL lifetimes (τav) of the inverted and normal cell were 1.277 and 1.94 ns, respectively. The diffusion coefficient (Dn) and charge carrier lifetime (τn) for the inverted MAPbI3 planar hybrid solar cells were increased by 1.14-fold and 1.1-fold, respectively, compared with the conventional FTO/TiO2/MAPbI3/PTAA:tBP + Li–TFSI/Au planar hybrid cells. Hence, the inverted MAPbI3 planar hybrid solar cells exhibited better power conversion efficiency and stability than the conventional MAPbI3 cells because (i) the electron extraction from MAPbI3 to the electron conductor was improved because the electron conductivity of PCBM is higher than that of TiO2; (ii) the EQE value was increased by the better charge injection/separation efficiency between MAPbI3 and PCBM, and by the higher charge collection efficiency than the conventional cell; (iii) the fill factor is improved by the increased Dn and τn; and (iv) the air and humidity stability is improved by the absence of corrosive additives in the device architecture and the hydrophobicity of the PCBM top layer. The reduced current density–voltage (J–V) hysteresis with respect to the scan rate and scan direction in the inverted planar hybrid solar cells could be attributed to a more balanced electron flux (Je) and hole flux (Jh), and a reduced number of surface traps.

Abstract: The DC conductivity ( σ DC ) of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(4-styrenesulfonate) (PSS) with various organic solvents was measured. The solvents used were dimethyl sulfoxide (DMSO), N , N -dimethyl formamide (DMF), tetrahydrofuran (THF), and H 2 O (as pristine solvent). Room temperature DC conductivity [ σ DC (RT)] of a free standing film of PEDOT/PSS with H 2 O was measured to be ∼0.8 S/cm. Through a change of solvents used, σ DC (RT) of the samples increases from ∼0.8 to ∼80 S/cm. The temperature dependence of DC conductivity [ σ DC ( T )] of PEDOT/PSS with H 2 O followed a quasi one-dimensional variable range hopping model, while that of PEDOT/PSS prepared from DMSO, DMF, and THF followed a power law ( σ DC ∝ T β ). From X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) experiments, the doping concentration of the systems with different solvents was approximately the same. We analyzed that the screening effect of the solvent plays an important role for the variation of σ DC of the PEDOT/PSS systems.

Abstract: Quantum random walks on graphs have been shown to display many interesting properties, including exponentially fast hitting times when compared with their classical counterparts. However, it is still unclear how to use these novel properties to gain an algorithmic speedup over classical algorithms. In this paper, we present a quantum search algorithm based on the quantum random-walk architecture that provides such a speedup. It will be shown that this algorithm performs an oracle search on a database of N items with $O(\sqrt{N})$ calls to the oracle, yielding a speedup similar to other quantum search algorithms. It appears that the quantum random-walk formulation has considerable flexibility, presenting interesting opportunities for development of other, possibly novel quantum algorithms.

Abstract: Highly conductive and transparent poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) films, incorporating a fluorosurfactant as an additive, have been prepared for stretchable and transparent electrodes. The fluorosurfactant-treated PEDOT:PSS films show a 35% improvement in sheet resistance (Rs) compared to untreated films. In addition, the fluorosurfactant renders PEDOT:PSS solutions amenable for deposition on hydrophobic surfaces, including pre-deposited, annealed films of PEDOT:PSS (enabling the deposition of thick, highly conductive, multilayer films) and stretchable poly(dimethylsiloxane) (PDMS) substrates (enabling stretchable electronics). Four-layer PEDOT:PSS films have an Rs of 46 Ω per square with 82% transmittance (at 550 nm). These films, deposited on a pre-strained PDMS substrate and buckled, are shown to be reversibly stretchable, with no change to Rs, during the course of over 5000 cycles of 0 to 10% strain. Using the multilayer PEDOT:PSS films as anodes, indium tin oxide (ITO)-free organic photovoltaics are prepared and shown to have power conversion efficiencies comparable to that of devices with ITO as the anode. These results show that these highly conductive PEDOT:PSS films can not only be used as transparent electrodes in novel devices (where ITO cannot be used), such as stretchable OPVs, but also have the potential to replace ITO in conventional devices.