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Sub-wavelength TiO2 nanospheres arranged in a two-dimensional (2D) square array and a 2D hexagonal array are used to increase the light-extraction efficiency of an air/GaN interface, which is theoretically investigated by using the finite-different time-domain method. The simulation results show that the 3-dB bandwidth of light extraction can be largely increased from 29 nm to 121 nm when the coupled periodically electric quadrupoles (CPEQ) is supported in a 2D square array. In addition, the near critical coupling of 99.4% can be achieved at the wavelength of 735 nm when the gap distance between adjacent TiO2 nanospheres is 110 nm. The manipulation of field orthogonality between the electric and magnetic fields results in an increase in the bandwidth of CPEQ, which plays an important role for the ultra-broadband and efficient light extraction.

We report on an efficient solar-heat shield based on the highly transparent PEDOT:PSS thin film, which significantly reduces the temperature from 38.2 C to 33.5 C. To understand the superior solar-heat shielding ability, the modified PEDOT:PSS thin films were investigated by using the transmittance spectra, Raman scattering spectra, atomic-force microscopic (AFM) images and broadband refractive index spectra. The experimental results show that the molecular structure of the PEDOT chains can be manipulated by varying the Brownian motion of PEDOT polymers in the PEDOT:PSS solutions and the spin rate. The lower Brownian motion of PEDOT polymers and the higher spin rate both can change the molecular structure of the PEDOT chains from bended structures (de-doping states) to linear structures (doping states), which simultaneously increase the near-infrared absorption strength and the visible transparency due to the formation of bipolarons.

A high-efficiency inverted-type CH3NH3PbI3 (MAPbI3) solar cell was fabricated by using a ultrathin poly[3-(4-carboxybutyl)thiophene-2,5-diyl]-Na (P3CT-Na) film as the hole transport layer. The averaged power conversion efficiency (PCE) can be largely increased from 11.72 to 18.92% with a double-filtering process of the P3CT-Na solution mainly due to the increase in short-circuit current density (JSC) from 19.43 to 23.88 mA/cm2, which means that the molecular packing structure of P3CT-Na thin film can influence the formation of the MAPbI3 thin film and the contact quality at the MAPbI3/P3CT-Na interface. Zeta potentials, atomic-force microscopic images, absorbance spectra, photoluminescence spectra, X-ray diffraction patterns, and Raman scattering spectra are used to understand the improvement in the JSC. Besides, the light intensity-dependent and wavelength-dependent photovoltaic performance of the MAPbI3 solar cells shows that the P3CT-Na thin film is not only used as the hole transport layer but also plays an important role during the formation of a high-quality MAPbI3 thin film. It is noted that the PCE values of the best P3CT-Na based MAPbI3 solar cell are higher than 30% in the yellow-to-near infrared wavelength range under low light intensities. On the other hand, it is predicted that the double-filtering method can be readily used to increase the PCE of polymer based solar cells.

The power conversion efficiency (PCE) of single-junction solar cells was theoretically predicted to be limited by the Shockley–Queisser limit due to the intrinsic potential loss of the photo-excited electrons in the light absorbing materials. Up to now, the optimized GaAs solar cell has the highest PCE of 29.1%, which is close to the theoretical limit of ~33%. To pursue the perfect photovoltaic performance, it is necessary to extend the lifetimes of the photo-excited carriers (hot electrons and hot holes) and to collect the hot carriers without potential loss. Thanks to the long-lived hot carriers in perovskite crystal materials, it is possible to completely convert the photon energy to electrical power when the hot electrons and hot holes can freely transport in the quantized energy levels of the electron transport layer and hole transport layer, respectively. In order to achieve the ideal PCE, the interactions between photo-excited carriers and phonons in perovskite solar cells has to be completely understood.

An Ag modified bathocuproine:ZnO nanoparticles (BCP:ZnO NPs) thin film was used as the buffer layer of inverted-type perovskite solar cells, which increased the bifacial power conversion efficiency from 6.82% to 15.50%. The surface-enhanced Raman scattering and absorbance spectra show that the Ag modified BCP small molecules can effectively passivate the surface oxygen defects of ZnO NPs and thereby increasing the crystallinity, which simultaneously increases the short-circuit current density (JSC) and fill factor. It is noted that a high incident photon-to-electron conversion efficiency (IPCE) of 92% is achieved at the wavelength of 587 nm due to the constructive interference effect in the multilayer structure, which can be used to explain the high photocurrent generation in a semi-transparent solar cell. In addition, the light intensity-dependent experimental results of the bifacial perovskite solar demonstrates that the hole transportation is better than the electron transportation in the perovskite thin film. This concept can be readily used in the optimization of high-efficiency bifacial perovskite solar cells.