Abstract
A physics-based explicit mathematical model for the external voltage-dependent forward dark current in bulk heterojunction (BHJ) organic solar cells is developed by considering Shockley-Read-Hall (SRH) recombination and solving the continuity equations for both electrons and holes. An analytical model for the external voltage-dependent photocurrent in BHJ organic solar cells is also proposed by incorporating exponential photon absorption, dissociation efficiency of bound electron-hole pairs (EHPs), carrier trapping, and carrier drift and diffusion in the photon absorption layer. Modified Braun’s model is used to compute the electric field-dependent dissociation efficiency of the bound EHPs. The overall net current is calculated considering the actual solar spectrum. The mathematical models are verified by comparing the model calculations with various published experimental results. We analyze the effects of the contact properties, blend compositions, charge carrier transport properties (carrier mobility and lifetime), and cell design on the current-voltage characteristics. The power conversion efficiency of BHJ organic solar cells mostly depends on electron transport properties of the acceptor layer. The results of this paper indicate that improvement of charge carrier transport (both mobility and lifetime) and dissociation of bound EHPs in organic blend are critically important to increase the power conversion efficiency of the BHJ solar cells.
Highlights
Bulk heterojunction (BHJ) polymer solar cells based on blends of conjugated polymers and fullerene derivatives (e.g., P3HT:PCBM blend) have drawn a huge attention in research due to their high conversion efficiency, solution-based easy fabrication, and abundant availability [1,2]
In our previous publication [8], we developed an explicit expression for the photocurrent in bulk heterojunction (BHJ) cells by incorporating exponential photon absorption, dissociation efficiency of bound electron-hole pairs (EHPs), carrier trapping/recombination, carrier drift and diffusion, and actual solar spectrum
We explicitly examine both the dark and net current behaviors as a function of the external voltage in BHJ organic solar cells by comparing the model calculations with recently published experimental results
Summary
Bulk heterojunction (BHJ) polymer solar cells based on blends of conjugated polymers and fullerene derivatives (e.g., P3HT:PCBM blend) have drawn a huge attention in research due to their high conversion efficiency, solution-based easy fabrication, and abundant availability [1,2]. -alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT) with the soluble fullerene derivative (PCBM) show nearly unity dissociation efficiency [4] Their cell efficiency strongly depends on the charge collection efficiency due to trapping/recombination of free carriers. In our previous publication [8], we developed an explicit expression for the photocurrent in BHJ cells by incorporating exponential photon absorption, dissociation efficiency of bound EHPs, carrier trapping/recombination, carrier drift and diffusion, and actual solar spectrum. We have developed a physics-based mathematical model for the external voltage-dependent forward dark current by considering SRH recombination in the active layer. Modified Braun model [11], which shows good agreement with the exact extension of Onsager theory except at extremely high electric field, is incorporated in the model to determine the dissociation of the bound EHPs. The analytical model is compared with the published experimental results in order to determine the carrier transport properties. We examine the effects of the contact properties, blend compositions, charge carrier transport properties, and cell design on the current-voltage characteristics
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