Probing Defect States in Organic Polymers and Bulk Heterojunctions Using Surface Photovoltage Spectroscopy

Abstract

We performed frequency-modulated (AC) and steady-state (DC) surface photovoltage spectroscopy (SPS) measurements on a bilayer structure consisting of an organic semiconductor (P3HT, P3HT:PC61BM, or PFBT2Se2Th:PC71BM) on top of a ZnO electron-transport layer. The AC spectra overlap with the absorption spectra of the organic layer, providing evidence that AC SPS corresponds to band-to-band transitions. The DC spectra are generally broader than the AC spectra, with responses extended below the absorption edge. Thus, DC SPS also probes transitions between band states and trap states within the band gap in addition to band-to-band transitions. When a hole-transport layer (HTL) is deposited on top of the organic layer, the DC spectra of P3HT and P3HT:PC61BM are narrower than those without the HTL, suggesting that the sub-band gap states exist at the surface of these organic semiconductors. In contrast, PFBT2Se2Th:PC71BM does not show signature of surface states or optically active trap states in the band gap. External quantum efficiency and capacitance measurements are employed to explain the nature of sub-band gap states that contribute to surface photovoltage signals and the differences between the two bulk heterojunction systems.

Understanding the Light‐Intensity Dependence of the Short‐Circuit Current of Organic Solar Cells

“…These deep defects states have been found in various types of organic solar cells originating from the energetic disorder of the materials and depending on factors like thermal annealing and prolonged illumination. [ 46–49 ] The recombination via these defects can be quantified by Shockley–Read–Hall recombination statistics. For trap states in the center of the bandgap, the recombination rate R SRH is given by Here, n is the electron concentration, p is the hole concentration and τ n/p is the charge‐carrier lifetime.…”

Device Architecture Study in Fullerene-Based Organic Photovoltaics

“…SPS results of 0, 1, 5, and 10 wt % P3HT in PC 71 BM on PEDOT:PSS and ZnO are shown in Figure , parts c and d, respectively. The positive SPS in Figure c and negative SPS in Figure d indicate that holes transfer to PEDOT:PSS and electrons transfer to ZnO, respectively. − SPS of 0 wt % P3HT on PEDOT:PSS (open red circles in Figure c) and on ZnO (solid red circles in Figure d) have the same threshold at 1.75 eV (∼708 nm), which corresponds to the PC 71 BM optical band gap (red solid line in Figure a and red dashed line in Figure b) . With increasing P3HT concentration, the thresholds of spectra for both transport layers shift to lower energies, i.e., below the PC 71 BM band gap.…”

“…With increasing P3HT concentration, the thresholds of spectra for both transport layers shift to lower energies, i.e., below the PC 71 BM band gap. While below band gap SPS signals can arise from charge transfer state absorption at donor–acceptor interfaces or midgap defect states, we believe it is due to charge transfer state absorption in these samples because this part of the SPS increases with the P3HT concentration, concomitant with increasing donor–acceptor interfacial area …”

Characterization of Strain-induced Defects Density in Flexible Organic Photovoltaic Cells using Capacitance Spectroscopy

“…Many researchers have studied and calculated the DOS in organic materials and devices using various methods such as Deep Level Transient Spectroscopy (DLTS) [18] and Thermally Stimulated Current (TSC) [19]. In addition, capacitance characterization has been popularly employed to measure not only the DOS [20,21] but also the doping density [22][23][24], built-in potential [21,24], carrier mobility [25] and even thermal stability [26] in different materials. Capacitance spectroscopy involves measurement of the depletion capacitance Cj over a range of biases and frequencies and finding the DOS by analyzing the capacitance-voltage, C(V), and capacitancefrequency, C(f), plots.…”

“…Thenceforth, this method has been applied to organic devices as well, including OPVs. For instance, characterization of the defect states in NPB [29], determination of the interfacial and deep defect states in organic solar cells [20,[29][30][31] and investigation of trap states formed in different deposition rates [32] or traps resulted from degradation [21,[33][34][35] are all conducted using capacitance spectroscopy in organic devices. It is worth noting that the techniques used for inorganic semiconductors are applicable to organic materials only when fitting models and parameters are employed due to the differences in band gaps, resistances and carrier mobilities.…”