The effect of carrier mobility in organic solar cells

Abstract

The microscopic states and performance of organic solar cell are investigated theoretically to explore the effect of the carrier mobility. With Ohmic contacts between the semiconductor and the metal electrodes there are two origins of carriers in the semiconductor: the photocarriers generated by photon absorption and the dark carriers diffused from the electrodes. The power efficiency of the solar cell is limited by the recombination of a carrier with either the photocarrier or a dark carrier. Near the short-circuit condition the photocarrier recombination in the semiconductor bulk decreases as the mobility increases. Near the open-circuit condition the dark carrier recombination increases with the mobility. These two opposite effects balance with one another, resulting in an optimal mobility about 10 −2 cm 2 / V s which gives the highest power conversion efficiency. The balance of the electron and hole mobilities are not necessary to maintain the optimal efficiency also because of the balance of the photocarrier and dark carrier recombination. The efficiency remains about the same as one carrier mobility is fixed at 10 −2 cm 2 / V s while the other one varies from 10 −1 to 10 −3 cm 2 / V s. For solar cell with a Schottky barrier between the semiconductor and the metal electrode there is no dark carrier recombination. The efficiency therefore always increases with the mobility.

Combined influence of carrier mobility and dielectric constant on the performance of organic bulk heterojunction solar cells

“…The influence of mobility on the efficiency and the existence of an optimum mobility have been well discussed in previous studies. [3][4][5][6] From Fig. 3, a higher dielectric constant ε r leads to a higher efficiency, thus a higher peak efficiency.…”

“…[3][4][5][6][7] It has been shown that there is an optimum value of the mobility that leads to a peak efficiency. [3][4][5][6] Mobilities lower than the optimum value lead to lower efficiencies due to significant reductions in the dissociation rate of polaron pairs into free charge carriers, 3 together with less efficient charge extractions. Mobilities above the optimum value lead to decreases in the open circuit voltage and no significant increase in the dissociation rate of polaron pairs.…”

Sensitivity of the Drift-Diffusion Approach in Estimating the Power Conversion Efficiency of Bulk Heterojunction Polymer Solar Cells

“…This gives a comprehensive picture of the influence of charge carrier mobility, layer thickness, exciton state, and loss mechanisms on the performance of the cell. In addition, it demonstrates how PCE estimation depends on physical parameters (e.g., mobilities [25][26][27]) and the assumptions made for the sub-macroscopic physical processes.…”

“…In order to determine the optimum mobility for OSC, several theoretical studies drive the impact of mobility on device performance [26,27,[29][30][31][32][33]. Concerning the Langevin theory, charge carrier mobility directly competes with the recombination, where the recombination increased for higher mobility.…”

“…However, in several studies, it was shown that the probability of charge carrier recombination relates to the mobility of the carrier. In fact, the expected recombination mechanism in low mobility organic materials, such as an active layer absorber of OSCs, is described by Langevin's theory [23][24][25][26][27][28][29][30][31][32] where the time for an electron and a hole to find each other defines the recombination rate. In the Langevin-type mechanism, the recombination coefficient and consequently the recombination rate is proportional to the electron and hole mobility which is described by 2 n p q μ +μ γ = ε .…”