Effect of Polymer Binders on UV-Responsive Organic Thin-Film Phototransistors with Benzothienobenzothiophene Semiconductor

The influence of polymer binders on the UV response of organic thin-film phototransistors (OTF-PTs) is reported. The active channel of the OTF-PTs was fabricated by blending a UV responsive 2,7-dipenty-[1]­benzothieno­[2,3-<i>b</i>]­[1]­benzothiophene (C5-BTBT) as small molecule semiconductor and a branched unsaturated polyester (B-upe) as dielectric binder (ratio 1:1). To understand the influence of the polymer composition on the photoelectrical properties and UV response of C5-BTBT, control blends were prepared using common dielectric polymers, namely, poly­(vinyl acetate) (PVAc), polycarbonate (PC), and polystyrene (PS), for comparison. Thin-film morphology and nanostructure of the C5-BTBT/polymer blends were investigated by means of optical and atomic force microscopy, and powder X-ray diffraction, respectively. Electrical and photoelectrical characteristics of the studied OTF-PTs were evaluated in the dark and under UV illumination with a constant light intensity (<i>P</i> = 3 mW cm^–2, λ = 365 nm), respectively, using two- and three-terminal <i>I–V</i> measurements. Results revealed that the purposely chosen B-upe polymer binder strongly affected the UV response of OTF-PTs. A photocurrent increase of more than 5 orders of magnitude in the subthreshold region was observed with a responsivity as high as 9.7 AW^–1, at <i>V</i>_G = 0 V. The photocurrent increase and dramatic shift of <i>V</i>_Th,average (∼86 V) were justified by the high number of photogenerated charge carriers upon the high trap density in bulk 8.0 × 10¹² cm^–2 eV^–1 generated by highly dispersed C5-BTBT in B-upe binder. Compared with other devices, the B-upe OTF-PTs had the fastest UV response times (τ_r1/τ_r2 = 0.5/6.0) reaching the highest saturated photocurrent (>10⁶), at <i>V</i>_G = −5 V and <i>V</i>_SD = −60 V. The enhanced UV sensing properties of B-upe based OTF-PTs were attributed to a self-induced thin-film morphology. The enlarged interface facilitated the electron withdrawing/donating functional groups in the polymer chains in influencing the photocharge separation, trapping and recombination

Transient Photovoltage in Perovskite Solar Cells: Interaction of Trap-Mediated Recombination and Migration of Multiple Ionic Species

It is highly probable that perovskite solar cells (PSCs) are mixed electronic-ionic conductors, with ion migration being the driving force for PSC hysteresis. However, there is much that is not understood about the interaction of ion migration with other processes in the cell. The key question is: what factors of a PSC are influenced when ions are free to move? In this contribution, we employ a numerical drift-diffusion model of PSCs to show that the migration of both anions and cations in interaction with trap-mediated recombination in the bulk and/or at the surfaces of the perovskite absorber can manifest both current–voltage hysteresis and unusual nonmonotonic PSC photovoltage transients. We identify that a key mechanism of this interaction is the influence of the net ionic charge throughout the perovskite bulkwhich varies as the ions approach new steady-state conditionson the distribution of electrons and holes and subsequently the spatial distribution of trap-mediated recombination modeled after Shockley Read Hall (SRH) statistics. Relative to intrinsic recombination mechanisms, SRH recombination can be highly sensitive to local asymmetries of the electron–hole population. We show that this sensitivity is key to replicating nonmonotonic transients with multiple time constants, the forms of which may have suggested multiple processes. This work therefore supports the conceptualization of the hysteretic behavior of PSCs as dominated by the interplay between ion migration and trap-mediated recombination throughout the perovskite absorber

Inverted Hysteresis in CH₃NH₃PbI₃ Solar Cells: Role of Stoichiometry and Band Alignment

J–V hysteresis in perovskite solar cells is known to be strongly dependent on many factors ranging from the cell structure to the preparation methods. Here we uncover one likely reason for such sensitivity by linking the stoichiometry in pure CH₃NH₃PbI₃ (MAPbI₃) perovskite cells with the character of their hysteresis behavior through the influence of internal band offsets. We present evidence indicating that in some cells the ion accumulation occurring at large forward biases causes a temporary and localized increase in recombination at the MAPbI₃/TiO₂ interface, leading to inverted hysteresis at fast scan rates. Numerical semiconductor models including ion accumulation are used to propose and analyze two possible origins for these localized recombination losses: one based on band bending and the other on an accumulation of ionic charge in the perovskite bulk