Mechanism of Organic Photovoltaics: History
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The power conversion efficiencies (PCEs) of organic photovoltaics (OPVs) have reached more than 19%, along with the prosperous development of materials and device engineering. 

  • organic photovoltaics
  • ternary strategy
  • bulk-heterojunction
  • power conversion efficiency

1. Introduction

Organic materials have the advantage of solution processing, low cost, light weight, flexibility, easy fabrication, and abundant material resources [1]. Organic photovoltaics (OPVs) promises to be one of the major solar energy conversion technologies in our daily life. The bulk heterojunction (BHJ) type OPVs have stood out and made great progress in increasing the power conversion efficiencies (PCEs) in recent years [2]. The BHJ structure refers to a solution-processed active layer formed using the blend acceptor and donor solutions. Acceptor and donor materials interlace with each other, forming a bicontinuous interpenetrating network nanostructure [3].

2. Working Mechanism of Organic Photovoltaics

The active layer is composed of a donor material and an acceptor material and is sandwiched between the hole and electron modification layer. The active layer of the BHJ structure is composed of a mixture of donor and acceptor materials. In an ideal BHJ, the two materials form a dual, continuous, and interpenetrating network with a large interface area and appropriate nanoscale phase separation, allowing for effective exciton separation and charge collection, respectively. 
(i)
Photon absorption and exciton generation. In order to improve the light capture efficiency of OPVs, the BHJ active layer should have a wide absorption spectrum and a high molar extinction coefficient. The introduction of electron-absorbing groups or the addition of conjugated acceptor materials can effectively reduce the lowest unoccupied molecular orbital (LUMO) level and band gap and broaden the absorption spectrum, increasing the short circuit current (JSC) in total [4].
(ii)
Exciton diffusion to the donor-acceptor (D-A) interface. Excitons diffuse in the donor or acceptor phase, dissociate to free charge carriers at the D-A interface, or decay back to the ground state by radiative or nonradiative pathways after excitons are produced. The free charge needs to reach the D-A interface completely in order to improve the efficiency of exciton diffusion. Proper domain purity and size are necessary for improved exciton diffusion efficiency (ηED) and high JSC. The blending film should have appropriate miscibility and crystallinity.
(iii)
Exciton dissociation. Excitons form charge transfer (CT) states at the D-A interface, which then completely dissociates into free electrons and holes. Excitons need to overcome the constraints of Coulomb forces. The difference in LUMO levels between the donor and acceptor effectively drives electron transfer. Regarding the mechanism of the D-A interface, the transition from the highest occupied molecular orbital (HOMO) level of the acceptor to the HOMO of the donor has been observed in non-fullerene acceptor (NFA)-based OPVs. The photons absorbed by the acceptor can also be used for photoelectric conversion, which helps increase the JSC of the OPVs. Even if the difference in HOMO between the donor and acceptor is close to zero, the hole transfer is still effective. This helps to reduce the open circuit voltage (VOC) loss.
(iv)
Charge carrier transport. After the exciton dissociation, the free charge carriers will move toward their respective electrodes. The efficiency of free charge movement depends on the morphology of the film and the charge mobility characteristics of the semiconductor materials. The introduction of the D-A structure can realize intermolecular charge transfer and increase the electron mobility of the acceptor material. JSC and the fill factor (FF) were improved. The appropriate levels of distortion and alkyl chains can effectively influence morphology to enhance the JSC and FF [4][5].
(v)
Charge carrier collection at individual electrodes. The holes and electrons arrive at the anode and cathode, respectively, forming an electric current. Charge collection efficiency (Pcoll) depends not only on the difference between the electrode and the D-A work function (WF), but also on the barrier at the organic compound/metal interface.
The efficiency of exciton diffusion depends on the size of the acceptor domain and the length of exciton diffusion. The size of the D-A domain is mainly determined by the morphology, while the diffusion length is controlled by the exciton lifetime (the coupling between the excited state and the ground state) and the relative probability of nonradiative decay. The charge transfer rate is determined by the coupling between the local excited state and the CT state, the energy shift between the local excited state and the CT state, and the recombination energy. The efficiency of CT exciton separation depends on the lifetime of the CT state, the coupling between the CT state and the charge separation (CS) state, the delocalization of electrons in the acceptor or holes in the donor, and the relative availability of the states.
The charge transport process is determined by the coupling between adjacent molecules, the degree of capture and desorption, and the lifetime of free charge carriers. Pcoll depends on the degree of interface defects, the size of the extraction potential barriers, and surface velocity. These operational processes can be described as the competition between charge transfer (transport after the generation of free charges) and recombination after light absorption.
Singlet exciton migration and dissociation compete with the radiative and nonradiative decay to the ground state. Comparing the photoluminescence efficiency or exciton lifetime of neat and blending materials is a common method to determine the exciton dissociation efficiency (ηCT) [6].

η d , e x = τ n e a t τ b l e n d τ n e a t

The lowest excited singlet state for either the donor or acceptor requires excitons to reach the D-A interface within their lifetime to ensure charge transfer before excitons leave the interface or decay to the ground state. The efficiency of exciton diffusion to the interface is closely related to the multiphase morphology (size and purity of individual domains) of the BHJ blends. The energy difference between the lowest excited singlet state on either donor or acceptor and CT states [7] is represented as follows:
ECT = ES1ECT1
ECT is the driving force of exciton dissociation (charge transfer). The recombination and charge transfer rates can be determined by the molecular structures, orientation, and interactions, as well as the interactions with the environment of the donors and acceptors, which are still challenging.

This is involved in the four main parameters during the processes of carrier collection efficiency (ηCC), ηCT, and ηED and the absorption efficiency (ηA). The external quantum efficiency (EQE) is a function of wavelength (λ), which can also be expressed as [6]

EQE(λ) = ηCC(λ) × ηCT(λ) × ηED(λ) × ηA(λ).
JSC is the maximum current flowing through the device, originating from the internal field. The process of photon collection plays an important role in photocurrent. The donor and the acceptor are mixed to form the active layer to achieve photon absorption complementarity. The absorption of photons can be enhanced by increasing the thickness of the active layer, introducing a third component, and synthesizing new acceptor materials with strong near-infrared absorption and high extinction coefficients. JSC is not solely reliant on the light collection ability of the active layer but rather is also influenced by exciton dissociation, charge transport, and the extraction processes that are microscopically determined. JSC is influenced by all photovoltaic processes, in which (1) the production of excitons is heavily reliant on the absorption coefficient and the band gap of the donor; (2) the diffusion of excitons is influenced by the electronic structure, dielectric constant, and donor morphology (phase size) of the conjugated block; (3) exciton dissociation is influenced by the electronic structure, dielectric constant, and energy level shifts (∆EHOMO and ∆ELUMO) between the donor and acceptor; (4) the transport and collection of carriers are determined by the hole mobility of the donor and its balance with the electron mobility of the acceptor. High permittivity can decrease the exciton binding energy and recombination events. If the donor phase exceeds a certain size, the excitons that are generated in regions where the distance from the D-A interface is greater than the length of exciton diffusion will undergo recombination, leading to a reduction in JSC.
In order to generate an external photocurrent, free charges must be extracted onto the electrode. This process is called diffusion or drift, which is represented by Pcoll. Extraction does not compete with free-charge recombination in pairs but is also affected by traps and extraction barriers. One manifestation of this competition is the FF. Regardless of the exact mechanism of free-charge formation and recombination, the externally measured photocurrent density (Jph) is the result of at least four processes, each competing with a specific loss process. They can be expressed as the product of survival efficiency relative to each loss mechanism:

Jph(F) = absƞd,exƞd,CT(F)ƞcollɸinc

in which ƞd,ex and ƞd,CT(F) refer to charge generation efficiency. Here, q refers to the charge, ɸinc refers to the incident photon flux density, and η refers to the probability of the incident photon being absorbed by the active layer. ƞd,CT and ƞcoll depend on the internal electric field, F, and determine the shape of the density-voltage (J-V) property.
VOC originates from the splitting of the electron and hole quasi-Fermi levels:

V OC = 1 e   ( E Fn E Fp )

in which EFp and EFn refer to the hole and electron quasi-Fermi levels, respectively. The VOC is determined by the difference between the ELUMO of the acceptor and the EHOMO of the donor, as well as by the dielectric constant of both the donor and acceptor. A lower EHOMO of the donor can lead to a higher VOC, while a larger dielectric constant can decrease the exciton binding energy and reduce the ∆ELUMO required for exciton dissociation, resulting in an increased VOC. The VOC is influenced by carrier density, light intensity and recombination, state density distribution, CT state, the microstructure of the blend, and the D-A interface area, which have limited impact on its determination.
The FF is determined by the competition between the recombination and extraction of free charge. It is defined as the ratio of the product of the VOC and JSC of the maximum power output (Pm) of OPVs:

FF = P m V O C × J S C = V m p × J m p V O C × J S C

where Vmp and Jmp refer to the voltage and current density at the maximum power point, respectively. The value of the FF is affected by the charge transport process between the photoactive layers. The FF is greatly influenced by the hole mobility (μh) of the donor and the equilibrium with the electron mobility (μe) of the acceptor. The morphology of D-A blends also exerts a certain impact on the FF. Carrier mobility is affected by various factors, such as crystallinity and crystal orientation, side chains, building blocks, membrane morphology, and the number of structural defects. Structural defects refer to regional irregular units, terminal groups, a disordered arrangement of monomers in the copolymers, branches, light crosslinked units, etc. Most structural defects are challenging to identify, as they can have detrimental effects on crystallinity, morphology, and charge transport, ultimately leading to poor battery performance. The important composite mechanism in OPVs is the surface composition of the contact point, which is to extract the wrong type of carrier. The surface compound reduces the FF and increases the nonradiation composite loss of VOC. The composition of the surface is created by the unintentional spread of the carrier to the wrong contact, and the harmful effect of this type of surface composition is the most prominent in the near-open road condition. The composition of the surface also affects the intensity dependence of VOC. In order to improve contact and reduce the surface composition, the electrode’s middle layer is usually used.
The main photovoltaic parameter PCE can be expressed as

P C E = J S C × V O C × F F P i n

Pin refers to the incident light power. The VOC mainly depends on the HOMO and LUMO levels. JSC is related to (i), (ii), (iii), (iv), and (v) mentioned above. The FF describes the “square” of the J-V curve, which represents the “difficulty” of extracting the photo-generated carriers from OPVs. Each of the basic processes occurring in OPVs possibly becomes a bottleneck, limiting the efficiency of OPVs. For OPVs with high efficiency, the factors are summarized as follows: (i) photon harvesting capacity: the coincidence degree of the wavelength of sunlight range and the absorption photon range of the active layer, as well as the ability to generate excitons; (ii) the appropriate energy level arrangement and active layer morphology can realize efficient exciton dissociation and charge transport; (iii) the interfacial engineering of charge collection.

This entry is adapted from the peer-reviewed paper 10.3390/en16114494

References

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