Effects of Effective Layer Thickness, Light Intensity and Electron-Hole Pair Separation Distance on The Performance of Organic Bulk Heterojunction Solar Cells
Subject Areas : Journal of Optoelectronical Nanostructures
1 - Department of Physics, Bojnourd Branch, Islamic Azad University, Bojnourd, Iran
Keywords: Organic Photovoltaic (OPV), Bulk Hetero-Junction (BHJ), Open Circuit Voltage, Short Circuit Current,
Abstract :
In this paper the influence of different parameters such as active layer thickness, light intensity and charge separation distance on the photocurrent-voltage, short circuit current density (Jsc) and open circuit voltage (Voc) characteristics in MEH-PPV:PCBM BHJ devicesis studied. For this purpose, the numerical continuum modelbased on drift-diffusion approximation is used. The J-V characteristics of MEH-PPV:PCBM BHJ devices under illumination change considerably with varying the active layer thickness from 40nm to 280nm. In these devices, as the active layer thickness increases from 40 nm to 120 nm the short-circuit current density increases dramatically. The open circuit voltage (Voc) is partially affected by varying the active layer thickness. In these devices, as the light intensity increases, the current density would increase at low voltages. Also, as the charge separation distance “a” increases, The exciton dissociation rate (kdissnexc) and current density would decrease.
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Islamic Azad University
| Journal of
Spring 2024 / Vol. 9, No. 2 |
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Research Paper
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Effects of Effective Layer Thickness, Light Intensity and Electron-Hole Pair Separation Distance on The Performance of Organic Bulk Heterojunction Solar Cells
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Aliasghar Ayobi*,1 1Department of Physics, Bojnourd Branch, Islamic Azad University, Bojnourd, Iran
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Received: 20 Feb. 2024 Revised: 28 Mar. 2024 Accepted:25 Apr. 2024 Published: 15 Jun. 2024
| Abstract In this paper the influence of different parameters such as active layer thickness, light intensity and charge separation distance on the photocurrent-voltage, short circuit current density (Jsc) and open circuit voltage (Voc) characteristics in MEH-PPV:PCBM BHJ devicesis studied. For this purpose, the numerical continuum modelbased on drift-diffusion approximation is used. The J-V characteristics of MEH-PPV:PCBM BHJ devices under illumination change considerably with varying the active layer thickness from 40nm to 280nm. In these devices, as the active layer thickness increases from 40 nm to 120 nm the short-circuit current density increases dramatically. The open circuit voltage (Voc) is partially affected by varying the active layer thickness. In these devices, as the light intensity increases, the current density would increase at low voltages. Also, as the charge separation distance “a” increases, The exciton dissociation rate (kdissnexc) and current density would decrease. |
Use your device to scan and read the article online
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Keywords: Organic Photovoltaic (OPV), Bulk Hetero-Junction (BHJ), Open Circuit Voltage, Short Circuit Current
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1. INTRODUCTION
During the last decade, much researches have been concentrated on organic photovoltaic (OPV) devices due to their many beneficial properties including low cost for material and fabrication equipment, lightweight, flexibility, renewable energy sources and room temperature solution process deposition. However, owing to their low power conversion efficiency (PCE), intensive research is done to improve their efficiency using new materials and device structures [1-3]. Bulk hetero-junction (BHJ) structure consists of an interpenetrating network of n-type (donor) organic material and p-type (acceptor) organic materials. Organic conducting materials have attracted much attention for use in organic BHJ solar cells due to their optical, electronic and mechanical properties and charge transfer between donor and acceptor organic materials [4-7]. However, these materials have the following limitations: the big band gap of these materials limits the ability to capture photons with wavelengths higher than the wavelength of sunlight and the very low charge carrier mobility of these materials leads to poor conductivity and cuts down the output power efficiency. Also, due to the possibility of changing properties of organic materials such as molecular mass, energy band gap and optical absorption, the performance of OPVS can be improved. It is possible to fabricate organic solar cells with higher variability and flexibility because a variety of novel absorber and transporter materials has been synthesized in the last decade. It has been reported that the power conversion efficiency of organic BHJ solar cells can be exceed than 11% [8,9].
For created electron-hole pairs after photo absorption, it might occur each of the following processes: Electrons and holes can be collected by the cathode or anode or recombine with each other. Cathode (anode) is as a right electrode for electron (hole) and anode (cathode) is as a wrong electrode for electron (hole). Only the electrons (holes) collected by the right electrode contribute to the photocurrent. In this paper considering ohmic contacts, charge carriers collected by wrong electrode are neglected and the most important factor for limiting the solar cell efficiency is charge carrier recombination. Recombination of each electron can occur with the photo-generated holes (photo-carrier recombination) or diffused holes from the anode (dark carrier recombination). In the short-circuit condition due to the high built-in potential, the dark carriers cannot diffuse in to the bulk of organic semiconductor and are concentrated in the organic-metal interface while in the open-circuit condition due to the existence of a flat energy band, the dark carriers diffuse inside the bulk.
The replacing of Ohmic contacts with Schottky contacts leads to the changes of the solar cell behavior. So that dark carriers are removed due to the energy barriers and the recombination is related to the photo-carriers. With increasing the carrier mobility, the solar cell efficiency increases due to the faster motion of carriers toward the electrode under built-in potential. In fact for schottky contacts, the collection of carriers with wrong electrode is not negligible since in this case there are not interface charge carriers while for ohmic contacts, electrons (holes) cannot reach to the high work function anode (cathode) due to the existence of high density of holes (electrons) around the interface between the effective material and anode (cathode). Therefore recombination will occur in the effective medium before reaching the interface.
The performance of organic solar cells is characterized with several device parameters.Since experimentally device optimization for obtaining highest efficiency is a challenging task, for this purpose the simulation based studies have key importance and despite a wide range of models and numerical simulations for OPVs, it is necessary to quantitatively describe the bulk and interface processes in these devices.During the last two decades, extensive researches have been done to investigate the physical mechanismsofconverting light into electricity in OPVS through the use of optical and electrical models for these devices.The most important subjects under investigation in these devices despite the progress in device physics are as follows: the origin of open circuit voltage and its dependence to light intensity and temperature, limits of the conversion efficiency, and origin of the recombination [10-12]. The physical mechanisms related to the temperature dependence of Voc are not fully clear. This temperature behavior can be related to thetemperature dependence of built-in voltage [13].
The efficiency oforganic BHJ solar cell is affected by four processes as follows: the absorption of photons and creation of excitons, exciton diffusion to the donor-acceptor interface, exciton dissociation to the free charge carriers in the hetero-junctions, and carrier transport toward the electrods [14]. In the organic BHJ solar cells, exciton dissociation is well done due to the large amounts of charge separation interfaces. Also for active layer thickness larger than 200 nm, the photon absorption reaches 90% [15]. Polymer photovoltaic devices with bulk-heterojunction (BHJ) structure have the highest power conversion efficiency. This isdue to the high surface contacts for charge separation and interpenetrating network for efficient charge transport [16-19]. In order to describe photovoltaic processes in OPVS, one can use either continuum or microscopic (discrete) model.Microscopic models based on Monte Carlo (KMC) simulations are used to study various processes in OPVS, including recombination rate-dependent efficiency, charge injection, interfacial recombination, interaction between charge carriers and electrodes, and etc [20-23]. However, the main problem of the KMC method is its high computational cost. In this paper the Continuum modelsbased on the drift-diffusion approach are used as a useful computational method to describe the performance of the organic BHJ solar cells based on the MEH-PPV: PCBM [24, 25].
2. MODEL AND EQUATIONS
In this paper, the physical processes of the organic BHJ solar cells such as, photocurrent-voltage characteristic are investigated by means of theoretical methods based on drift-diffusion approximation and a simulation tool ATLAS-SILVACO package. For this purpose the influence of different parameters such as active layer thickness, light intensity and charge separation distance on the photocurrent characteristics, short circuit current (Isc) and open circuit voltage (Voc) in MEH-PPV:PCBM BHJ devices is studied with a numerical device model.
In this model, the effective material approximation is considered for drift-diffusion simulations, in which any real interface between acceptor and donor materials are removed from simulation and the blend is considered as a single homogeneous material.In the effective medium approximation the ionization potential (IP) which defines the valance state energy (hole transport level) of the film is determined by the highest occupied molecular orbital (HOMO) level of the donor. Also the electron affinity (EA) which defines the conduction state energy (electron transport level) is determined by the lowest unoccupied molecular orbital (LUMO) level of the acceptor. The energy gap of effective material( is defined as difference between these levels (figure 1). The main advantage of the effective medium approach is possibility of one –dimensional simulation of single layer BHJ device located between two metal contacts [26-35]. Moreover in this study, several approximations are considered as follows: parabolic density of states similar to what is considered for inorganic crystalline materials, space charge effects, recombination effect, ohmic contacts for charge collection, poll-Frankel mobility models, and neglecting losses due to exciton quenching [36, 37].
as Langevin bimolecular recombination rate constant, as exciton binding energy, J1 as the first order Bessel function and as field parameter. In these equations, q denotes the electronic charge, denotes the material’s dielectric constant, denotes the permittivity of free space and kBdenotes the Boltzmann’s constant.
In disordered polymer systems, the charge-separation distance is not constant. Therefore, the overall exciton dissociation probability is optained with using a spherically averaged Gaussian distribution as follows [49]:
(4)
In this equation, parameter “a” denotes the charge-separation distance in the conditions that the probability of the Gaussian function is maximum value.
Of course, because of the controvercial presecnce of the long range electric field within the effective medium, there are doubts about the accuracy of this model to describe exciton dissociation in the effective medium approximation.Therefore, several authors with neglecting drift mechanism have considered a completely diffusion driven mechanism for charge transport in effective medium. However, due to the presence of the electric field in donor-acceptor interface one can assume that in the limit of the effective medium, this interface electric field which is the reason of the exciton splitting is substituted with a bulk electric field.
C. Basic equations
This simulation is based on the Drift diffusion equations which can be used to describe the free charge transport in organic materials. In this model, The electron (Jn) and hole (Jp) current densities are related to the carrier densities (n,p) and the electrostatic potential (:
(5)
(6)
In these equations, q is elementary charge, and μp are the electron and hole mobility respectively and Dn and Dp are the electron and hole diffusion coefficients respectively which are characterized through the Einstein relation with T as absolute temperature and kB as Boltzmann’s constant:
(7)
(8)
The carrier densities in thermal equilibrium condition are given as:
(9)
(10)
With Nc and Nv as the effective densities of states (DOS) of LUMO and HOMO respectively, , Ec and Ev as the energy of the conduction band (LUMO level) and valance band (HOMO level) respectively and EF as the Fermi level.
Drift-diffusion equations are coupled to the continuity equation for charge conservation and poisson equation to include the electrostatic potential:
(11)
(12) 
(13)
The Poisson equation relates the electrostatic potential to the electron (n(x)) and hole (p(x)) densitiesso that, and are ionized acceptor and donor densities respectively, q is the elementary charge and 
is the dielectric constant. In continuity equations, Jn(x) and Jp(x) are the electron and hole current densities respectively, G is defined as the optical generation rate of free electron-hole pairs from exciton dissociation and Rn(Rp) is the total recombination rate of electrons (holes).
The charge transport in disordered organic materials is described conventionally by Poole-Frankel (PF) type field dependent hopping mobility model. According to this conventional model, at low electric field limit, carrier mobility is dependent to the temperature only but with increasing the applied electric field, it will be dependent to the applied electric fieldas follows:
Where 
, E=-d
/dx and E0 are the mobility in the limit of zero field, electric field and the characteristic field of the materials respectively. Empirically, the temperature dependences of the field activation factor (
) and is often found to be well described by:
(16)
Where 
=0.5ev, T0=500k and the mobility in the limit of zero field and infinite temperature (
and B are separate adjustable parameters for electrons and holes [50, 51].
In literature, the direct recombination rate is used as dominant mechanism for describing recombination in most organic BHJ solar cells:
(17)
In this equation 
and 
exp[
is recombination constant and intrinsic charge carrier density respectively. According to the Langevin theory is given as follows:
With 
as the permittivity of the material [26, 44, 52, 53]. The planar OPVs devices under investigation in this manuscript have very small thickness, therefore only one spatial dimension (x) is considered.
In equations 12 and 13, the local exciton density nexc is coupled to drift diffusion equations characterizing free charge carrier transport through the G=kdissnexc. With neglecting exciton transport in BHJ devices, this local exciton density is calculated through a local rate equation:
(19)
In this equation, Goptical and kdissnexc describe the constant generation rate of excitons with respect to the position through the sunlight absorption and the exciton dissociation rate respectively, kdecnexc describes the exciton recombination rate through the radiative and non-radiative processes and 
is the generation rate of excitons equal to the free electron-hole pair recombination process (
.
D. Organic-metal interface and charge extraction
In the final step of OPVs performance, the free charge carriers must be collected with the contacts. The open circuit voltage of the OPV is controlled by the work-functions of the materials composing the two contacts. Therefore the investigation of contact/effective medium interface is an important factor. There are several models for organic/metal interfaces in contactsthat in which the presence of trap states at the interfaces are considered to influence the charge injection from the contact into the organic layer. However these trap states have little effect on the extraction of photo-generated carriers in the organic/metal interfaces of OPVs and can be neglected. With considering a constant vacuum level between organic layer and metal interface and neglecting the dipole shift related to the charge accumulation at the interface, only the work-function of the metal at the contacts affects on the energy alignment between energy gap and metal Fermi energy. In this simulation study a BHJ device is considered to be sandwiched between two metal contacts as anode and cathode that are located at x=0 and x=d respectively (figure1(a)). Also, a simple temperature dependent injection model (thermionic injection model) is used for describing the current or the charge carrier densities at the contacts [20, 54]:
=
(20)
In this equation 
(
and n0represent the interface recombination velocity and the equilibrium charge carrier density respectively with Nc as effective density of states and 
as injection barrier for electrons defined as difference between the charge transport levels and the metal work function. For organic material with parabolic density of states similar to the inorganic material with standard Schottky model this velocity is related to the effective mass of the semiconductor. The metal/organic contact of BHJ solar cell, is considered to be a sufficient majority charge carrier extraction. In this case 
and thus the contacts are in equilibrium and all excess charge carriers are instantaneously extracted (n(d)=n0(d) and p(0)=p0(0)). If some insulating layer be placed between metal and active material, the majority carrier extraction velocity will reduced. This topic is not discussed in this manuscript.
Often, the concepts “majority” and “minority” are related to the doped layers. In this manuscript, these concepts are related to the concentrations of charge carriers in active layer of BHJ device close to the contacts. Therefore electrons and holes are considered as majority charge carrier at the cathode and anode respectively provided that 
. The equilibrium densities of electrons at the anode and holes at the cathode are very low due to the large injection barrier as (for electrons at the anode and (
for holes at the cathode. Therefore these carriers are defined as the minority.
The built-in potential (Vbi) is defined as the difference between anode and cathode work functions and according to the Fig.2 can be written as follow:
(21)
To obtain a proper solution for the problem it is necessary to apply the appropriate boundary condition. If the work function of the anode (PEDOT:PSS) lies below the HOMO level of MEH-PPV and the work function of cathode (Ca) lies above the LUMO level of PCBM it would be possible to consider the Ohmic boundary conditions (Figure 1(b)).
In this case, for the anode located at x=0:
(23)
And for the cathode located at x=L:
n(L)=
(24)
(25)
Another boundary condition is applied to the electric potential in the short circuit condition as follows:
The necessary parameters for this simulation are given in table 1[55-58].
TABLE 1. The quantities used in device simulation.
Quantity | Symbol | Value | ||
Dielectric constant |
| 3.4 | ||
Relaxation rate | kr |
| ||
Exciton charge-separation distance | a | 1.3 nm | ||
Electron transport level (PCBM LUMO) | Ec | 3.7 eV | ||
Electron effective density of states | Nc | 5.3 | ||
Hole transport level (MEH-PPV HOMO) | Ev | 5.07 eV | ||
Hole effective density of states | Nv | 4.2 | ||
Electron zero-field mobility |
|
| ||
Electron Poole-Frenkel field parameter |
| 1 | ||
Hole zero-field mobility |
|
| ||
Hole Poole-Frenkel field parameter |
| 4 |
Citation: Aliasghar Ayobi. Effects of effective layer thickness, light intensity and electron-hole pair separation distance on the performance of organic bulk heterojunction solar cells. Journal of Optoelectronical Nanostructures. 2024; 9 (2): 22- 46. DOI: 10.30495/JOPN.2024.32692.1305 |
|
Address:Department of Physics, Bojnourd Branch, Islamic Azad University, Bojnourd, Iran. Tell:09151863185 Email: auobi_ali@yahoo.com |
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