Enhancement of charge carrier transport by doping PVK-based photoconductive polymers with ${\text{LiNbO}}_3$ nanocrystals

The photoconductive properties of a hybrid material based on poly(N-vinylcarbazole) (PVK) doped with nanoparticles of lithium niobate $\left({\text{LiNbO}}_3\right)$ have been studied. The experiments were carried out using modulated photocurrent and non-steady-state photo-emf techniques at 633 nm wavelength. The photoconductivity of the composite increases remarkably with the nanoparticle content. This growth is related to larger drift lengths of the photoholes at higher nanoparticle concentrations. The reason for this enhancement is an increase in the effective photohole lifetime due to trapping of electrons on the nanoparticles and creation of recombination barriers at the polymer/nanoparticle interface.

Figure 1

(Color online) Experimental dependence of the ac photocurrent $J_{\text{ph}}^{\Omega }$ $\left(\Omega /2\pi =1\text{Hz},I_0=0.95\text{W}/{\text{cm}}^2,R_L=1\text{M}\Omega \right)$ on the dc field $E_0$ for different concentrations of ${\text{LiNbO}}_3$ NPs: ◻ 0; ◼ 0.001; ● 0.1; ▲ 1, ▼2, ◆ $5\text{wt}\%$.

Figure 2

(Color online) Dependence of the photocurrent $J_{\text{ph}}^{\Omega }$ on the modulation frequency $\Omega /2\pi$ $\left(E_0=3\text{V}/\mu \text{m},I_0=0.95\text{W}/{\text{cm}}^2,R_L=1\text{M}\Omega \right)$ for different concentrations of ${\text{LiNbO}}_3$ NPs ◼ 0.001, ▲ 1; ◆ $5\text{wt}\%$ normalized to the zero-frequency photocurrent value. The straight lines describe the asymptotic behavior at low and high frequency.

Figure 3

(Color online) Modulated photocurrent $J_{\text{ph}}^{\Omega }$ vs the average light intensity $I_0$ $\left(\Omega /2\pi =1\text{Hz},E_0=2\text{V}/\mu \text{m},R_L=1\text{M}\Omega \right)$ in the sample which contains $5\text{wt}\%$ of ${\text{LiNbO}}_3$ NPs.

Figure 4

(Color online) Non-steady-state photo-emf current $J_{\text{p-emf}}^{\Omega }$ vs the external dc field $E_0$ for different concentrations of ${\text{LiNbO}}_3$ NPs ◼ 0.001, ▲ 1; ◆ $5\text{wt}\%$ $\left(\Omega /2\pi =2\text{Hz},I_0=0.95\text{W}/{\text{cm}}^2,R_L=10\text{M}\Omega \right)$. The photo-emf signal is normalized to the zero-field value. The solid lines are guides for the eyes.

Figure 5

(Color online) Concentration dependence of the normalized photocurrent amplitude $J_{\text{ph}}^{\Omega }/J_{\text{ph}R}^{\Omega }$ (at $E_0=3.5\text{V}/\mu \text{m}$ and $\Omega /2\pi =1\text{Hz}$) and of the normalized drift lengths $L_0/L_{0R}$. Here $J_{\text{ph}R}^{\Omega }$ and $L_{0R}$ are the values of photocurrent and drift length measured in the reference material without ${\text{LiNbO}}_3$ NPs.

Figure 6

[(a)–(d)] Energy scheme of the polymer: NP composite for different NP electron affinities; (e) electron affinity is the same as in case (d), but now the LUMO layer of the sensitizer is lowered.

Figure 7

Band bending effect and creation of the drift and recombination barriers in the polymer-nanocrystal composite under illumination.