Ethynyl-linked porphyrin hetero-dimers substituted by a series of electron donors, namely, bis(4-methoxyphenyl)amino (BMPA), bis(4-tert-butylphenyl)amino (BTBPA) and 3,6-di-tert-butylcarbazol-9-yl (DTBC) as well as a reference dimer with a non-donor moiety (3,5-di-tert-butylphenyl, DTBP) have been synthesized to systematically investigate the influence of donor introduction on the photovoltaic performances of near-IR dye-sensitized solar cells (DSCs) with these sensitizers incorporated. Despite the expected bathochromic shift and intensification of long-wavelength absorption bands as well as elevated LUMO levels and thus increased electron injection driving forces, the substitution of diphenylamino groups (BMPA and BTBPA) with stronger electron-donating abilities gave rise to surprising mediocrity in the short-circuit photocurrent densities (J(sc)), leading to overall energy conversion efficiencies in the order BMPA (3.94%) < DTBP (4.57%) < BTBPA (4.83%) < DTBC (5.21%). A study of the in situ fluorescent behavior of these sensitizers revealed that for all the sensitizers, excited-state lifetimes were significantly shortened in the simulated DSC environment compared to those in a free solution. BMPA showed the shortest intrinsic in situ lifetime while DTBC showed the longest one. These results were correlated with the photovoltaic performances, which is required for a better understanding and further design of porphyrin array sensitizers.

Tandem solar cells that combine perovskite (PVK) top cells and Si, Cu(In,Ga)(Se,S) 2 (CIGS), and other bottom cells have attracted much attention for increasing the efficiency of solar cells. To use the PVK solar cells as the top cells, their metal electrode needs to be replaced with a transparent conductive layer such as indium tin oxide (ITO) deposited by sputtering, where thermally evaporated MoO x needs to be introduced to protect the underlying hole transport layer (HTL) from sputtering damage. In this study, it was revealed that the effect of ion bombardment during ITO sputtering on widely used spiro-OMeTAD was not detrimental to the device performance, whereas it improved the power conversion efficiency (PCE) owing to the better band alignment caused by oxidation of the HTL. By eliminating the use of the MoO x buffer layer, we were able to develop semitransparent PVK cells with higher durability and transmittance. PCEs of 19.5% (certified 19.3%) and 26.2% were achieved for a 1 cm 2 buffer-free semi-transparent PVK cell and four-terminal PVK/CIGS tandem solar cells, respectively.

The highest theoretical efficiency of double junction solar cells is predicted for architectures with the bottom cell bandgap (E g ) of approximately 0.9-1.0 eV, which is lower than that of a typical Si cell (1.1 eV). Cu(In,Ga)(Se,S) 2 (CIGS) solar cells exhibit a tunable E g depending on their elemental composition and depth profile. In this study, various CIGS solar cells with E g ranging from 1.02 to 1.14 eV are prepared and a spectrum splitting system is used to experimentally demonstrate the effect of using lower-E g cells as the bottom cell of two-junction solar cells. The four-terminal tandem cell configuration fabricated using a mixed-halide perovskite top cell (E g = 1.59 eV; stand-alone efficiency = 21.0%) and CIGS bottom cell (E g = 1.02 eV; stand-alone efficiency = 21.5%) with a 775-nm spectral splitting mirror exhibits an efficiency of 28.0% at the aperture area of 1 cm 2 .

Out of the scientific concern on the kinetics versus energetics for rational understanding and optimization of near-IR dye-sensitized solar cells (DSCs), an N-fused carbazole-substituted ethynyl-linked porphyrin heterodimer (DTBC) reported previously by our group was focused upon in terms of photovoltaic, photoelectrochemical, and steady-state and time-resolved photophysical properties in varied electrolyte environments. A primitive attempt to balance the photocurrent against the photovoltage by varying the concentration of the common coadsorbent 4-tert-butylpyridine (TBP) revealed that TBP continuously suppressed injection but provided inadequate compensation in open-circuit voltage (V oc). This further drew out the perspective of the widely ignored dye–electrolyte interaction in DSCs, specifically the axial coordination of TBP to the central zinc cation in porphyrin sensitizers that may retard electron injection. As an alternative, a TBP-free electrolyte containing guanidinium thiocyanate was developed to realize greatly promoted V oc with little current sacrifice, thus significantly enhancing overall energy conversion efficiencies. The excited state was protracted to counteract the injection retardation caused by much reduced driving force, setting a successful example of bilateral compromise between kinetics and energetics in near-IR DSCs.

There are few reports on photoelectric conversion efficiency using naturally-occurring dyes for dyesensitized solar cells (DSSC). This is because the matching with an excited electronic level of naturally-occurring dye to the conduction band of semiconductor is problematic; the excited electrons are easily relaxed to the steady state with fluorescence or heat emission. We examined the fluorescence inhibition effect of a self-assembled photonic crystal using Chlorine e6 dye. Chlorine e6 is derived from chlorophyll and has a long excited electron lifetime. We prepared TiO 2 inverse opals with various particle sizes by liquid phase deposition and described their effect on DSSCs with regard to structural, optical and electrochemical properties. In addition, we explored the implications of fluorescence lifetime measurements relative to the photonic band diagram and the amount of adsorbed dye. Although the main factor affecting the external photoelectric conversion efficiency was the diffusion resistance of the electrolyte and the contact resistance between TiO 2 interfaces, the possibility that the dye fluorescence lifetime, i.e. the photonic band structure, can affect the internal quantum efficiency per one dye molecule was also investigated.

The photovoltaic characteristics (short circuit current, open circuit voltage, fill factor, and photon-to-electron conversion efficiency) and electrochemical impedance measurements of normal and inverse-opal dye-sensitized electrodes were carried out in a transparent electrolyte (0.6 M triethhanolamine/0.5 M lithium perchlorate in acetonitrile) using three different methods; electrophoretic deposition, TiCl4 coating, and liquid-phase deposition (LPD). X-ray diffraction (XRD) analysis revealed that each electrode was composed of anatase TiO2. The impedance elements discussed previously were substrate resistance (R0), interface resistance at indium tin oxide (ITO)/titanium oxide (TiO2) (R1), contact resistance between TiO2 particles (R2), interface resistance at TiO2/dye/electrolyte and counter electrode/electrolyte (R3), and diffusion resistance of the electrolyte (R4). The internal resistances of inverse opal electrodes were varied with the preparation methods, especially in relation to the contact resistance between R2 and R3. The electrode prepared by LPD exhibited the smallest internal resistances and the highest photon-to-electron conversion efficiency among the three methods.

Organometal halide perovskites (OHPs) have garnered considerable attention as materials for next-generation solar cells. Although the power conversion efficiencies of solar cells with OHPs are comparable to those of the existing solar cells, their durability must be improved for practical use based on a deep understanding of their basic properties. During the last decade, various methods, such as composition control, alkali metal doping, and surface passivation, were used to increase the durability of OHPs. In this study, the effects of K-doping on the thermal stability of mixed OHPs are investigated. The activation energies of the mixed OHPs with and without K-doping are found to be 128.4 ± 5.2 and 92.9 ± 9.8 kJ/mol, respectively. Furthermore, the Johnson–Mehl–Avrami model is adopted to the in situ X-ray diffraction measurement data, the results of which indicate that K-doped OHPs can be stabilized to such an extent that they do not decompose after 10,000 h of heating at 85 °C.