Reducing losses in solution processed organic solar cells
Rahimichatri, Azadeh
DOI:
10.33612/diss.170159026
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Publication date: 2021
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Rahimichatri, A. (2021). Reducing losses in solution processed organic solar cells. University of Groningen. https://doi.org/10.33612/diss.170159026
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1
Chapter 1
1
1.1 Renewable energy
In the developed world, the rapidly rising energy demand (average annual growth rate of 1.5%, since 2008 [1]) requires rapid increase in energy generation. The currently avail-able natural resources of energy are fossil fuels (coal, gas, oil), nuclear and renewavail-ables. Fossil fuels have been and are still remaining the largest energy resource for transporta-tion, electricity and heat production.[2] The dominant role of fossil fuels in our daily lives has come at the expense of the increase in greenhouse gas emissions in the atmosphere, which has made the planet warmer.[3, 4] The need for a global transition away from fos-sil fuels has resulted in an agreement within the United Nations Framework Convention on Climate Change (UNFCCC), the Paris agreement, whose objective is to hold the global temperature increase well below 2◦C.[5] Using large quantities of low carbon power
gen-eration sources while meeting the expanding energy demand, is an important challenge over the upcoming years to reduce concentration of greenhouse gases to sustainable lev-els. Renewable energy plays a key role in meeting that challenge.
There are several types of renewable energy resources such as the sun, wind, water, biomassandgeothermal. Anannualenergyof1.5×1018kWhisobtainedfromthesunon earth, which is about 28000 times the total primary energy consumption of the world.[6] The enormous potential of renewable energy sources can meet the world’s energy de-mand and bring the world back to a sustainable path.
Photovoltaic technology converts solar radiation into electricity, involving semicon-ducting materials and is based on the photovoltaic effect.[7] The first silicon solar cell was developed in 1954 by Chopin et al., with an efficiency of 6%.[8] Photovoltaic solar energy conversion has been commercial for decades, and is currently flourishing. In 2018, the world record for solar cell efficiency of 47.1% has been achieved by using multi-junction concentrator solar cells.[9] Perovskite solar cells have reached an efficiency of 25.5%, competing with single crystal silicon solar cells with a record efficiency of 26.1% (Figure1.1) Hybrid perovskite/silicon tandem solar cells with a certified power conver-sion efficiency of 29.15% have been also reported.[10] Using an efficient donor-acceptor copolymer donor, a power conversion efficiency of 18.22% has been achieved for single-junction organic solar cells, which is the highest efficiency for organic solar cells until January 2020.[11] Despite great advances in the field of organic photovoltaics, in order to compete with their inorganic counterparts and get into the market, organic solar cells must be improved further, both efficiency and stability wise.
1.2 Organic semiconductors
Organic semiconductors are carbon-based materials with exceptional optical and elec-tronic properties that make them suitable for organic elecelec-tronic applications. Organic
1
1.2. Organic semiconductors 3
Figure 1.1: Chart of photovoltaic research-cell efficiency versus years. This plot is courtesy of the
National Renewable Energy Laboratory, Golden, CO.
semicondutors offer advantages over their inorganic counterparts, such as solution pro-cessability and the possibility of manufacturing organic electronic devices on flexible substrates. Furthermore, their physical and chemical properties can be tuned by modi-fying their chemical structure. Semiconducting behavior in organic molecules/polymers originates from sp2hybridization of carbon atoms and the formation of alternating sin-gle and double bonds, which is called conjugation.[12]
A single carbon atom has a valence electron configuration of (2s)2(2p
x)1(2py)1(2pz)0. In sp2hybridization state, the 2s and two 2p-orbitals form three hybrid orbitals, leaving unchanged the last pzorbital. In a linear chain of sp2-hybridized carbon atoms, the three
sp2hybrid orbitals lie in a plane and form three σ bonds with neighboring atoms. The left over adjacent pzorbitals overlap in a side-by-side fashion to form π bonds, in which the electrons are delocalized. In ethylene (C2H4) with two carbon atoms, the electrons in the π bond are constrained to the region between the two carbon atoms. In conjugated chains consist of more than two carbon atoms, the π electrons are delocalized over the whole length of the molecule, which gives rise to charge carrier conducting properties in these systems.
The bonding of atoms in conjugated molecules is described by molecular orbital the-ory, where linear combinations of the atomic orbitals is used to form molecular orbitals of bonding or anti-bonding character. In a bonding orbital with lower energy, the elec-tron density is concentrated between two pairs of atoms, and therefore the atoms are held together. In an anti-bonding orbital, the electrons are more localized in the atomic
1
Figure 1.2: Transition from four isolated 2pzatomic orbitals of carbon atoms to four π
molec-ular orbitals of increasing energy for 1,3-butadiene (C4H6). The two bonding π orbitals in the
HOMO energy band are lower in energy than the atomic orbitals, while antibonding π* orbitals in the LUMO energy band are higher in energy.
orbitals, resulting in nuclear repulsion and higher energy.
The interaction among several atoms results in a series of molecular orbitals which get energetically closer as the number of atoms involved increases. As a result, bonding states with a maximum energy corresponding to the highest occupied molecular orbital, HOMO, and anti-bonding states with the lowest unoccupied molecular orbital, LUMO, are formed (see Figure 1.2). The energy gap between HOMO and LUMO (Eg) determines semiconducting properties and the ability to absorb light.[13–15]
1.2.1 Transport of charges in organic semiconductors
Charge transport in an organic semiconductor is controlled by intermolecular transfer of an electron (hole) in the LUMO (HOMO) of a molecule or a conjugated chain, to the empty LUMO (HOMO) of an adjacent site.[16]
The mobility of charges in disordered materials strongly depends on electric field, following a Poole-Frenkel behaviour:[17–19]
µ = µ0exp(γ
√
F ), (1.1)
where µ0is the zero-field mobility, F denotes the electric field strength, and γ is a tem-perature dependent parameter called field activation parameter. In conjugated organic films, disorders, induced by conformational freedom, inevitable variation in conjuga-tion length, chemical impurities, etc., cause variaconjuga-tion in site energies.[20] In such sys-tems, charge carriers are localized and charge transport therefore proceeds by hopping from one site to another. For a downward hop, the excess energy is dissipated in the
1
1.2. Organic semiconductors 5
form of phonons, while for an upward hop the energy is supplied by the absorption of a phonon.[16] The energetic and spatial disorder in organic semiconductors limit the intermolecular charge transport, which is characterized by a lower mobility of charge carriers compared to inorganic semiconductors where charge transport occurs via band transport mechanism.
Miller and Abrahams modeled the phonon assisted hopping rate Wijas[21]
Wij= ν0exp(−2γRij)
exp(− kj− BTi) i< j
1 i≥ j (1.2)
where ν0is the attempt-to-jump frequency, Rijis the distance between the states i and
j, γ is the inverse localization length, iand jare site energies, kBis Boltzmann’s
con-stant, and T is temperature. The first exponential term represents the transfer rate due to electronic coupling between adjacent sites, and is related to the degree of wavefunction overlap between them. The second exponential term corresponds to the temperature dependence of the phonon density.
One of the well-known models to describe hopping transport in disordered organic systems was introduced by Bassler.[22] In his model, a gaussian distribution for varia-tion in site energies was assumed, and a temperature and field dependent mobility was proposed.[22] However, it was only at high electric fields that the model agreed with the available experimental data.[23] The model was further improved by Gartstein and Conwell, who could successfully describe the transport of charges in molecularly doped polymers,[24]conjugatedpolymers,[25,26]andotherorganicsystems.[27]Thiswasdone by adding the contribution of spatial correlations between site energies,[28] which re-sulted in[24, 29] µ = µ∞exp 3σ 5kBT 2 + 0.78 σ kBT 3 2 − Γ qaF σ , (1.3)
where q is the elementary charge, a is the intersite spacing, σ is the standard deviation of Gaussian distribution, and Γ is the positional disorder of transport sites.
1.2.2 Measuring the charge carrier mobility
A simple, reliable method to characterize the charge transport in organic semiconduct-ing devices is space charge limited current (SCLC), with which the mobility of charge carriers can be determined under steady state conditions.[30–34] The SCLC method is based on probing injection currents to study insulators.[35] Upon injection of electrons from a metal to an insulator, with a negligible density of free charge carriers, the electrons flow in the conduction band of the insulator.[35] The presence of injected electrons, and their space charge, limits the injection of further electrons. It is thus the space charge in
1
the bulk of the semiconductor that limits the current flowing through it. The SCL current density is written as
JSCL= 9 8 µ
Vint2
L3, (1.4)
where is the dielectric constant of the material, Vintis the internal voltage drop across the layer, and L is the thickness. Considering Equation 1.1, JSCLis approximated by
JSCL= 9 8 µ0exp(0.891γ Vint L ) V2 int L3. (1.5)
The internal voltage drop across the semiconducting layer is related to the applied voltage Vaby
Vint= Va− Vbi− VRs, (1.6)
where the built-in voltage Vbiis the internal potential drop in thermal equilibrium while no external voltage is applied, and it arises from the difference in work function of the electrodes. Vbiis determined as the voltage at which the current-voltage characteristic becomes quadratic, where the device enters SCL regime. VRsis the voltage drop across the series resistance of the semiconductor (typically 30-40 Ω).
In order to extract the electron or hole mobility using SCLC measurements, a single carrier device structure is required. The electrodes are chosen such that the work func-tion of one electrode is close to the LUMO (HOMO) of the acceptor (donor) to inject elec-trons (holes) into the semiconducting film, while the other electrode blocks injection of holes (electrons).
1.3 Photovoltaic devices
A typical current-voltage (J − V ) curve of a solar cell is shown in Figure 1.3. Several pa-rameters are used to characterize the performance of a solar cell, which are determined from its J − V characteristic under illumination. The current, per unit area, that flows in the external circuit when no voltage is applied to the solar cell is called short circuit current density (JSC). The voltage at which no current flows in the circuit is open circuit voltage (VOC). FF is the ratio between the maximum power generated by a solar cell and the product of JSCwith VOC.
F F = JMPPVMPP
JSCVOC (1.7)
The power conversion efficiency (PCE) is calculated as the ratio between the maxi-mum generated power of a solar cell and the incident light power (Pin). PCE is measured
1
1.4. Organic solar cells 7
Figure 1.3: Typical current-voltage characteristic of a solar cell under illumination, showing the
JSC, VOC, andFF. Blueareacorrespondstothemaximumpoweroutputofthesolarcell(JMPPVMPP).
under standard test conditions, where the incident light irradiance is 1000 W/m2for air mass 1.51
P CE = JSCVOCF F
Pin (1.8)
1.4 Organic solar cells
An organic solar cell is a type of photovoltaic device that comprises an organic photoac-tive layer and two electrodes. Many of the acphotoac-tive layers consist of donor and acceptor materials which can either be planar (single/multi-junction) or intermixed (bulk hetero-junction).
The generation of photocurrent in an organic solar cell involves several steps as de-picted in Figure 1.4. When the active layer absorbs light, an electron is excited from the HOMO to the LUMO level of the absorber. In contrast to inorganic solar cells, where free charge carriers are directly generated upon light absorption, in organic solar cells a tightly bound electron-hole pair, called exciton, is formed. An exciton has a binding energy of between 0.5 eV and 1 eV which is much higher than thermal energy at room temperature (26 meV).[37] This means that an alternative dissociation process is needed to separate electron and holes to extract a photocurrent and avoid exciton decay to its
1Air mass refers to the relative path length of the direct solar beam through the atmosphere. When the sun
is directly overhead, the path length is AM 1.0.[36] AM1.5 is the spectrum of sunlight after passing through 1.5 times the thickness of Earth’s atmosphere.
1
Figure 1.4: Schematic of five physical processes in an organic solar cell at MPP. Upon absorption
of light an exciton is generated (1), which diffuses towards the donor-acceptor interface (2). At the interface, the electron transfers to the acceptor molecule while the hole stays in the HOMO of the donor, and a bound electron-hole pair (charge transfer state) may be formed (3), which is followed by further dissociation and formation of free charge carriers (4). Free charge carriers are then transported to the electrodes (5). The blue spheres represent an electron, the red spheres represent a hole.
ground state, which typically takes place on nanosecond time scales.[38, 39] In most or-ganic semiconductors, exciton diffusion length is limited to 10 nm.[40, 41] If an acceptor material stays within the diffusion length of an exciton, and dissociation of the exciton is energetically favorable, the electron transfers to the acceptor molecule while the hole stays in the HOMO of the donor. In a single/multi junction organic solar cell with lay-ered structure, diffusion of excitons limits the performance of donor/acceptor bilayer, as only excitons that are generated close to the electron donor-acceptor interface con-tribute to the free charge generation process. Effective exciton dissociation is provided by a bulk heterojunction,[42, 43] where the initimate mixing of donor and acceptor fa-cilitates charge transfer at the donor/acceptor interface and the generation of free chare carriers before excitons decay. Free charge carriers are then transported to the electrodes to be extracted.
1.4.1 Loss mechanisms
The free charge carriers may undergo loss mechanisms, which limits the performance of organic solar cells. The bimolecular recombination rate was first derived by Langevin for ionsinagas.[44]Hismodeldescribesrecombinationdynamicsoftwooppositelycharged ions in a large reservoir, where the reference frame is attached to the electron and the pos-itive charge is moving. When the pospos-itive charge approaches below the coulomb capture radius, where thermal energy equals coulomb binding energy, the charges can no longer avoid recombination.
1
1.4. Organic solar cells 9
The flux density of holes crossing the sphere with radius rc, while an electron is at the center, is written as[44]
J = pvd= pµE = p(µn+ µp) q 4πr2
c, (1.9)
where vdis the drift velocity of holes. The recombination current is the current density flowing into the sphere of radious rcaround the electron. The total recombination flux can be written as
I = 4πr2cnJ = np(µn+ µp)q
, (1.10)
where n and p are the number of electrons and holes present per unit volume. On the other hand, under steady state conditions, the bimolecular recombination rate is de-scribed by the equation[45]
r = βnp, (1.11)
where β is the recombination coefficient. By comparing Equations 1.10 and 1.11, the Langevin recombination coefficient γLis written as[44]
γL=q
(µn+ µp). (1.12)
Inthermalequilibrium, therateofrecombinationequalstherateofgeneration. There-fore, the equilibrium rate equation is:
R0= G0= βn0p0 = βn2
i. (1.13)
Under steady state conditions, the rate of recombination is given by Equation 1.11. Therefore, the net recombination is obtained as
R = r− G0= β(np− n2
i). (1.14)
In bulk heterojunction solar cells, factors such as an inhomogeneous distribution of charge carriers may cause lower probability of the oposite charges finding each other and recombine.[46] Several studies have shown that depending on the active layer morphol-ogy, the preparation conditions and the organic materials, a deviation from the Langevin recombinationoccurswhichaddsanadditionalterm, γpre, totheLangevinexpression:[47– 50]
R = γpreγL(np− n2i) (1.15)
Most polymer:fullerene bulk heterojunction systems studied to date have a γpre be-tween 0.01 and 1.[48, 50–55] There are some reports for P3HT:PCBM solar cells showing a
1
γpreas low as 10−3, [47, 56] and for polymer:nonfullerene bulk heterojunction solar cells as low as 10−4. [57]
The presence of traps adds another recombination channel, which is described by the Shockley-Read-Hall (SRH) equation: [58, 59]
RSRH = CnCpNt
Cn(n + n1) + Cp(p + p1), (1.16) where RSRHis the rate of trap-assisted recombination, Cn(p)is the capture coefficient of electrons (holes), Nt is the density of charge traps, and n1 and p1 are parameters that introduce the dependency of the RSRHon the trapping energy level Etas
n1= Ncexp(Et− Ec
kBT ), (1.17)
and
p1= Nvexp(Ev− Et
kBT ). (1.18)
In equations 1.17 and 1.18, Ncand Nvare the effective density of states in the conduc-tion and valence bands, respectively, and Ecand Evare the energies of the conduction and valence bands.
Furtheremore, in a bulk heterojunction solar cell with ohmic contacts, the contacts induce charges (electrons at the cathode, holes at the anode) in the semiconductor ad-jacent the contacts.[60] Under operating conditions of a bulk heterojunction solar cell (Va<VOC), these charges can also participate in recombination processes close to the con-tacts and limit the device performance. The part of the current at the electrodes that is carried by the minority carriers is related to the density of minority charges as:[61, 62]
Jn(p)= qSn(p)[n(p)− n(p)eq] (1.19) where Jn(p)is the electron (hole) current at the anode (cathode), Sn(p)is the surface re-combination velocity, and n(p)eqis the equilibrium carrier density at the contact.
At open circuit, the current of the solar cell under illumination equals zero. Hence, under this condition, the photogeneration of charge carriers is completely canceled by recombination. This implies that the open-circuit voltage is limited by the amount of recombination that is present in the device.[63] Therefore, a reduction in the recombi-nation strength would, next to an increase in JSCand FF, lead to a higher VOC.
1.5 Doping in organic photovoltaics
A key property of an optoelectronic device is the transport of charge carriers through a thin film. Minimizing losses due to a poor charge transport is therefore an effective
1
1.5. Doping in organic photovoltaics 11
Figure 1.5: Schematic of the doping process for p-type (left) and n-type (right) doping in organic
semiconductors. In n-type (p-type) doping, the dopant is a donor (acceptor).
way for producing high performance devices. In most applications, highly conductive electron or hole charge transport layers are desirable, which is achieved by doping.[64– 67]
In inorganic semiconductors, doping involves substitution of an atom or addition of an interstitial atom within the crystalline matrix. These impurity atoms introduce ex-tra electrons or holes below the conduction band (n-type doping), or above the valence band (p-type doping) of a semiconductor, which become free carriers by being thermally ionized.[68] In organic semiconductors, n-type (p-type) doping is achieved by donation (extraction) of electrons to (from) the LUMO (HOMO) state of the host material, which requires sufficient energetic overlap of the host matrix and dopant to create free electrons and holes (see Figure 1.5).[67, 69]
n-type or p-type doping have been widely used to control energy levels, and improve charge transport and selectivity of electrodes in many organic electronic devices, includ-ing organic solar cells, organic light emittinclud-ing diodes, organic memristors, organic ther-moelectrics and hybrid organic-inorganic perovskite solar cells.[64, 70–81] n-type dop-ing is more challengdop-ing than p-type dopdop-ing due to the fact that the HOMO energy level of the dopant is located above the LUMO level of the host material, which means n-type dopants usually have low ionization potential and can rapidly oxidize in air.[67] Further-more, the low doping efficiencies of n-doped solution processed organic films has made development of organic n-doping progressively more important. Understanding the lim-iting factors of conductivity in n-doped solution processed organic semiconductors is still a topic of debate.
1
1.5.1 Stability of doped organic films
Long-term stability is one of the important factors determining the performance of many organic electronic devices. In such devices, maintaining high performance, mainly, un-der application of bias voltage, thermal stress, and air is crucial. However, some organic electronic devices suffer from being unstable. For instance, planar vacuum deposited perovskite solar cells using doped organic charge transport layers for a better extraction of charges, are less stable than those with undoped charge extraction layers.[82] There-fore, investigating ways to improve stability of doped organic films is of great importance. In contrast to doped inorganic semiconductors, dopants used for organic materials are not usually covalently bonded to the host, and therefore strongly tend to diffuse.[83] This causes the doped organic transport layers to be unstable under operating condi-tions, i.e., under a bias voltage stress. n-doped organic films that are highly tolerant to air oxidation can also offer cost beneficial fabrication of thin films.
1.6 Outline of this thesis
In spite of great progress in recent years, widespread application of organic photovoltaic devices requires further improvement of efficiency and stability. Considering the mul-tilayered structure of organic photovoltaic devices and the multiple parameters playing role in limiting their performance, maintaining their efficiency over time is a complex topic. This thesis aims at reducing losses in organic photovoltaic devices, first by quan-tifying recombination losses, and next, by studying doped charge transport layers.
• In Chapter 2, by means of simulations and new experimental tools, the contribu-tion of induced charges near the electrodes in recombinacontribu-tion rates of organic solar cells is studied. It is shown that in organic solar cells with very weak bimolecular recombination strength (γprelower than 10−3) induced charges can reduce device performance.
• In Chapter 3, the FF of nonfullerene acceptor organic solar cells is connected with transport properties and recombination of charge carriers using a simple experi-mental tool, which is supported by drift-diffusion simulations of organic solar cells, published earlier.[84]
• In Chapter 4, we provide signatures of efficient and inefficient doping in n-doped films of fullerene based host molecules, by making a distinction between presence of single and double ionic species in the film.
• In Chapter 5, the focus is on finding those host material properties that can en-hance the stability of organic electronic devices incorporating n-doped films of
1
1.6. Outline of this thesis 13
fullerene derivatives. It was revealed that, a fullerene derivative with short ethy-lene glycol side chain, namely PDEG-1, offers stable doped organic semiconduct-ing films under application of electrical and thermal stress.
1
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