Surface and Interface Properties in Thin-Film Solar Cells: Using Soft X-rays and Electrons to Unravel the Electronic and Chemical Structure

Record-High Electron Mobility Exceeding 16 cm2 V- 1 s- 1 in Bisisoindigo-Based Polymer Semiconductor with a Fully Locked Conjugated Backbone

Polymer semiconductors with mobilities exceeding 10 cm² V^{- 1} s^{- 1} , especially ambipolar and n-type polymer semiconductors, are still rare, although they are of great importance for fabricating polymer field-effect transistors (PFETs) toward commercial high-grade electronics. Herein, two novel donor-acceptor copolymers, PNFFN-DTE and PNFFN-FDTE, are designed and synthesized based on the electron-deficient bisisoindigo (NFFN) and electron-rich dithienylethylenes (DTE or FDTE). The copolymer PNFFN-DTE, containing NFFN and DTE, possesses a partially locked polymeric conjugated backbone, whereas PNFFN-FDTE, containing NFFN and FDTE, has a fully locked one. Fluorine atoms in FDTE not only induce the formation of additional CH∙∙∙F hydrogen bonds, but also lower frontier molecular orbitals for PNFFN-FDTE. Both PNFFN-DTE and PNFFN-FDTE form more ordered molecular packing in thin films prepared from a polymer solution in bicomponent solvent containing 1,2-dichlorobenzene (DCB) and 1-chloronaphthalene (with volume ratio of 99.2/0.8) than pure DCB. The two copolymers-based flexible PFETs exhibit ambipolar charge-transport properties. Notably, the bicomponent solvent-processed PNFFN-FDTE-based PFETs afford a high electron mobility of 16.67 cm² V^-1 s^-1 , which is the highest electron-transport mobility for PFETs reported so far. The high electron mobility of PNFFN-FDTE is attributed to its fully locked conjugated backbone, dense molecular packing, and much matched LUMO energy level.

A novel energy level detector for molecular semiconductors

The multifunction of molecule-based devices is always achieved by improving their charge transport characteristics. These characteristics depend strongly on the energy levels of molecular semiconductors, which fundamentally govern the working principle and device performance. Therefore, an accurate measurement of these energy levels is crucial for evaluating the availability of the prepared materials and thus optimizing the device performance. Here, an easy-to-operate three-terminal hot electron transistor has been developed, which comprises a molecular optoelectronic device that records the charge transport. It achieves exceptional properties including the lowest unoccupied molecular orbit level, highest occupied molecular orbit level, higher energy states, and higher electronic bandgap. When compared with existing techniques such as cyclic voltammetry, inverse photoemission spectroscopy, and ultraviolet photoemission spectroscopy, the hot electron transistor provides in-situ characterization and categorizes the measured energy information as intrinsic properties of the molecular semiconductor. Furthermore, we provide an in-depth understanding of the fundamental device-physics, which provides promising guidance for performance optimization.

In2O3:H-Based Hole-Transport-Layer-Free Tin/Lead Perovskite Solar Cells for Efficient Four-Terminal All-Perovskite Tandem Solar Cells

Narrow-band gap (NBG) Sn-Pb perovskites with band gaps of ∼1.2 eV, which correspond to a broad photon absorption range up to ∼1033 nm, are highly promising candidates for bottom solar cells in all-perovskite tandem photovoltaics. To exploit their potential, avoiding optical losses in the top layer stacks of the tandem configuration is essential. This study addresses this challenge in two ways (1) removing the hole-transport layer (HTL) and (2) implementing highly transparent hydrogen-doped indium oxide In₂O₃:H (IO:H) electrodes instead of the commonly used indium tin oxide (ITO). Removing HTL reduces parasitic absorption loss in shorter wavelengths without compromising the photovoltaic performance. IO:H, with an ultra-low near-infrared optical loss and a high charge carrier mobility, results in a remarkable increase in the photocurrent of the semitransparent top and (HTL-free) NBG bottom perovskite solar cells when substituted for ITO. As a result, an IO:H-based four-terminal all-perovskite tandem solar cell (4T all-PTSCs) with a power conversion efficiency (PCE) as high as 24.8% is demonstrated, outperforming ITO-based 4T all-PTSCs with PCE up to 23.3%.

Efficient passivation of n-type and p-type silicon surface defects by hydrogen sulfide gas reaction

An efficient surface defect passivation is observed by reacting clean Si in a dilute hydrogen sulfide-argon gas mixture (<5% H₂S in Ar) for both n-type and p-type Si wafers with planar and textured surfaces. Surface recombination velocities of 1.5 and 8 cm s^-1are achieved on n-type and p-type Si wafers, respectively, at an optimum reaction temperature of 550 °C that are comparable to the best surface passivation quality used in high efficiency Si solar cells. Surface chemical analysis using x-ray photoelectron spectroscopy shows that sulfur is primarily bonded in a sulfide environment, and synchrotron-based soft x-ray emission spectroscopy of the adsorbed sulfur atoms suggests the formation of S-Si bonds. The sulfur surface passivation layer is unstable in air, attributed to surface oxide formation and a simultaneous decrease of sulfide bonds. However, the passivation can be stabilized by a low-temperature (300 °C) deposited amorphous silicon nitride (a-Si:N_X:H) capping layer.

Sulfate Speciation Analysis Using Soft X-ray Emission Spectroscopy

The chemical and electronic structures of 15 different sulfates are studied using S L_2,3 soft X-ray emission spectroscopy (XES). Sulfur L_2,3 XES spectra of sulfates are distinctively different from those of other sulfur compounds, which makes XES a powerful technique for sulfate detection. Furthermore, subtle but distinct differences between the spectra of sulfates with different cations are observed, which allow a further differentiation of the specific compound. Most prominently, the position and width of the emission from "S 3s" derived bands systematically vary for different compounds, which can be understood with electronic structure and spectral calculations based on density functional theory.

Electronic Structure of the CdS/Cu(In,Ga)Se2 Interface of KF- and RbF-Treated Samples by Kelvin Probe and Photoelectron Yield Spectroscopy

Ambient-pressure Kelvin probe and photoelectron yield spectroscopy methods were employed to investigate the impact of the KF and RbF postdeposition treatments (KF-PDT, RbF-PDT) on the electronic features of Cu(In,Ga)Se₂ (CIGSe) thin films and the CdS/CIGSe interface in a CdS thickness series that has been sequentially prepared during the chemical bath deposition (CBD) process depending on the deposition time. We observe distinct features correlated to the CBD-CdS growth stages. In particular, we find that after an initial CBD etching stage, the valence band maximum (VBM) of the CIGSe surface is significantly shifted (by 180-620 mV) toward the Fermi level. However, VBM positions at the surface of the CIGSe are still much below the VBM of the CIGSe bulk. The CIGSe surface band gap is found to depend on the type of postdeposition treatment, showing values between 1.46 and 1.58 eV, characteristic for a copper-poor CIGSe surface composition. At the CdS/CIGSe interface, the lowest VBM discontinuity is observed for the RbF-PDT sample. At this interface, a thin layer with a graded band gap is found. We also find that K and Rb act as compensating acceptors in the CdS layer. Detailed energy band diagrams of the CdS/CIGSe heterostructures are proposed.