Since the first sucessful demonstration of organic heterojunction solar cells by Tang,[19] the power conversion efficiencies of organic solar cells have reached values comparable to their inorganic thin film analogs.[7] The term organic solar cell covers a wide range of different devices,[20-21] from all-polymer systems[22-24] through polymer-small molecule blends[25-27] to solar cells comprising only small molecules[28-30] and even polymer/inorganic hybrid systems.[31]
1.1.1.1 Architecture
A typical organic solar cell device is schematically shown in Figure 1-3a, comprising a glass substrate that is covered with a transparent indium tin oxide (ITO) front contact and poly(3,4-ethlyenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as a hole extraction layer. The active layer consists of a distinct donor and an acceptor material which can be assembled in various ways (vide infra). The back contact is a metal electrode that is evaporated on top of the organic layer stack. Common materials include calcium, aluminium and silver depending on the desired work function. An interlayer may be added between the active layer and back contact in order to facilitate efficient electron extraction.
1.1.1.2 Working principle
The working principle is shown in Figure 1-3, where the inset of Figure 1-3a gives information about the local sites of the individual steps, Figure 1-3b gives information about the electronic levels of the individual materials in general and the electronic frontier orbital energies of donor and acceptor in particular. Figure 1-3c finally illustrates the state energies that can be observed from excitation to final charge separation. Organic solar cells are excitonic systems in which light is absorbed (step 1) and an electron of the absorbent material (usually the donor) is excited from the ground state (S0) to the next excited state (S1) which results in a strongly bound electron-hole pair,[33] the frenkel exciton.[34]
Figure 1-3. (a) Schematic architecture of an organic solar cell device comprising a bulk heterojunction as the active layer. An electron extracting interlayer may optionally be inserted between the active layer and the back contact. The inset depicts the working principle for excitonic solar cells; (b) Electronic scheme of an organic solar cell with energy levels of the individual materials, HOMO is the highest occupied molecular orbital and can be estimated the ionisation potential (IP), LUMO is the lowest unoccupied molecular orbital and can be estimated from the electron affinity (EA); (c) State energy diagram illustrating the ground state (S0), state of the singlet exciton (S1), the charge transfer state (CT), also referred to as coulombically bound polaron pair (BPP) and the charge separated states (CS), 𝑬𝒆𝒙𝒄𝒃 is the binding energy of the singlet exciton, 𝑬𝑩𝑷𝑷𝒃 is the equivalent binding energy of the BPP states, ∆𝑬𝑪𝑺 is the enthalpy difference driving charge separation and is defined as the difference in enthalpy between the singlet exciton energy (ES1) and the enthalpy of the charge separated polarons at their respective material band edges (given by 𝑰𝑷 − 𝑬𝑨), ∆𝑮𝑪𝑺 is the total energy loss during the overall charge separation process;
(b-c) Adapted from Dimitrov et al.[32] Copyright 2014 American Chemical Society.
This exciton has diffusion lengths in the order of 10-20 nm[35-37] in which it has to reach a donor-acceptor interface (step 2). At the interface a charge transfer state (CT) is generated, which is also referred to as coulombically bound polaron pair (BPP). Charge separation can
1)
happen at the interface, if the energy available for charge separation is greater than the exciton binding energy (∆𝐸𝐶𝑆 > 𝐸𝑒𝑥𝑐𝑏 ). If this is the case, charges are separated (CS, step 3) leading to an electron in the acceptor and a hole in the donor material.[38-39] The electron (EFe) and hole (EFh) quasi-Fermi levels that are found in donor-acceptor blend during device operation are different from the band edges of the neat materials and their splitting corresponds to the free energy of photogenerated charge carriers after thermal relaxation (CSTR in Figure 1-3c). The charges are then transported (step 4) through the respective materials to the electrodes, i.e.
the hole through the donor material to reach the ITO front contact and the electron through the acceptor material to come to the metal back contact. Electrode interlayers, such as the depicted conducting PEDOT:PSS at the front contact and for example lithium fluoride (LiF) or zirconium acetylacetonate (ZrAcac) at the back contact can be used to adjust the electrode work functions or the built-in potential in the device and thus facilitate the extraction of holes and electrons, respectively.
1.1.1.3 Active layer morphology
In order to obtain highly efficient solar cell devices, the morphology of the active layer must meet specific demands. First, donor and acceptor should be intermixed to a level that excitons can diffuse to a donor-acceptor interface within their lifetime, i.e. within their exciton diffusion length of 10-20 nm. Second, both the donor as well as the acceptor material have to form a interconnected network without the formation of isolated domains in order to avoid charge recombination and to allow efficient charge transport to the respective electrodes. Last, the morphology should be thermally and temporally stable in order to allow high temperature processing in large scale production (e.g. accelerated ink drying in inline ovens) and to achieve long device lifetimes.
Figure 1-4. Active layer morphologies.
The active layer of the solar cell can be assembled in various ways and the most important ones are shown in Figure 1-4. Planar heterojunctions, i.e. bilayers, were used in the early days[19]
and are limited by the small exciton diffusion lengths for thick active layers and by inferior absorption in layers thin enough for efficient charge generation. However, this morphology also has advantages such as the easy processability of multilayer stacks (e.g. for energy level cascades[40-41] or tandem and multijunction solar cells[42-43]) and is, therefore, often used in industry. In particular for vacuum processable low molecular weight compounds, planar heterojunctions enable complex device stacks that in fact led to the latest certified power conversion efficiency record in OPV devices (12%, see section 1.1.2.4).[44]
bilayer (planar heterojunction)
blend (bulk heterojunction)
blockcopolymer (not aligned)
blockcopolymer (aligned: lamellar)
In 1995 the concept of bulk heterojunctions was first demonstrated,[45-46] tremendously simplifying the preparation of efficient active layers by mixing donor and acceptor materials followed by solution processing of this blend. Depending on processing parameters such as conentrations, solvents, solvent additives as well as annealing, an ideal morphology of finely mixed donor and acceptor domains exhibiting interconnected networks can be obtained. This approach is not only feasible for blending polymers with small molecules, but also for blending different polymers.[47] Due to the ease of processing, this system has become the most prominent morphology in academic OPV research, albeit severe limiting factors such as morphological instability at elevated temperatures: Even when the donor and acceptor domain sizes can exactly be tuned by processing, the resulting morphology is simply frozen upon evaporation of the solvent in thin film. This morphology is, however, not thermodynamically stable and thermal stress leads to a macrophase separation in the large majority of blends which leads to a vast deterioration of OPV device performance.[48]
One approach to stabilize the bulk-heterojunction morphology of an active layer is crosslinking of the active layer, either by use of crosslinkable low-bandgap polymers or by addition of external crosslinkers.[49-54] This solution is attractive especially for industry, as it does not unfavorably influence large scale processing and can be easily implemented in already available systems.[55]
A more advanced path is offered by blockcopolymers,[56-57] which cannot undergo macrophase separation as the donor and acceptor blocks are covalently linked. They are, however, known to microsphase separate into well defined morphologies (e.g. lamellar, see Figure 1-4) at length scales that match the exciton diffusion length.[58-62] Apart from representing the thermodynamic minimum, thus being thermally stable, these morphologies can offer close to perfect pathways for charge extraction.[63-67] However, a desired orientation of domains needs to be achieved to realize highly efficient devices.
The importance of morphology control in donor-acceptor systems has been underrated for a long time in the pursuit of materials with optimal energy levels. More recently, research aimed at developing tailored morphologies, well-suited for charge generation and transport in organic solar cells, was proposed to be at least as fruitful.[68-69]
1.1.1.4 Basic characterization
Organic solar cells are commonly characterized by their I-V characteristics (Figure 1-5a) and their external quantum efficiency (EQE, Figure 1-5b). During irradiation with light that matches the intensity and spectrum of AM1.5G sunlight, a counter voltage is applied to the photovoltage and the current is measured as a function of applied voltage.[70]
Figure 1-5. Typical characteristics of a solar cell: (a) I-V characterization with open circuit voltage VOC, short circuit current Isc, fill factor FF, maximum power point MPP and the voltage and current at the MPP VMPP and IMPP, respectively; (b) external quantum efficiency (EQE).
The electronic parameters used for characterization are the open-circuit voltage VOC, the short-circuit current density ISC and the fill factor FF. VOC is mainly determined by the difference of the donor HOMO and the acceptor LUMO and is the maximum voltage that can be obtained.
In most cases 0.6 eV loss is registered as energy requirement to create the charge separated state.[71] ISC is the maximum photocurrent density that is obtained under short-circuit conditions. The maximum power point MPP is defined as the point where the power output of the solar cell under continuous operation is highest. The fraction of the power obtained at this point (red shaded square) and the theoretically maximum calculated from the product of JSC and VOC (dashed green line) is defined as FF (see eq. 1), giving a measure of the quality of charge carrier extraction, which depends on different resistances and recombination effects.
= MPP MPP
OC SC
V I
FF V I (1)
The power conversion efficiency η is defined as the ratio of power produced by the device (Pout) and the power of the incident light (Pin).
η = out = OC SC
in in
P V I FF
P P (2)
The external quantum efficiency (EQE) is given by the number of electrons extracted from the device
-e out,
N divided by the number of incident photons Nphotons in, (see eq. 3) and is usually measured in dependence of the incident wavelength (see spectrum in Figure 1-5b) which gives information about the photocurrent contribution at different wavelengths arising from different materials, such as the donor and the acceptor.
= Ne out-, = ISC ⋅ λ
EQE N P hc (3)
(a) (b)
MPP
VOC
ISC IMPP
VMPP FF
maximal photo current