Shaobing Xiong is currently an associate professor in School of Physics and Electronic Science at East China Normal University. He received his B.S. in Physics from Soochow University in 2017 and Ph.D. in Physical Electronics from East China Normal University in 2022. His research focuses on the interface properties and photoelectric regulation of organic and perovskite optoelectronic devices.
Mats Fahlman received his M.S. in Engineering Physics (1991) and Ph.D. in Surface Physics and Chemistry (1995) from Linköping University, Sweden, where he is currently a full professor at the Laboratory of Organic Electronics. He is active in the research of organic semiconductors using primarily surface science techniques to explore materials and interfaces as well as their effect on device properties.
Qinye Bao is currently a professor in School of Physics and Electronic Science at East China Normal University. He received his Ph.D. in 2015 in Surface Physics and Chemistry from Linköping University, Sweden. The focus of his work is on surface science techniques, such as ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and near edge X-ray absorption fine structure spectroscopy, to reveal the relationship between interface electronic structures and device performance, especially for application in organic solar cells, organic light-emitting diodes, and perovskite-based optoelectronic devices.Improving the quality of life for Earth’s growing population is a complex task that requires the development of new technologies and materials. Perhaps the biggest challenge is access to clean and renewable energy sources that can drive a sustainable future. Photovoltaics, today mainly represented by silicon-based solar cells, convert solar energy into electricity and is already an important component in the renewable energy portfolio. Organic solar cells (OSCs) and perovskite solar cells (PSCs) both offer advantages compared to silicon solar cells such as low-temperature solution processing, flexibility and semi-transparency while sharing similar device structures consisting of a photoactive layer (PAL) sandwiched between an anode and a cathode contact. The anode and cathode contacts in OSCs and PSCs typically consist of a charge transport layer (CTL) and a conductor (often a metal), and the CTLs significantly impact device performance. The CTLs used in OSCs and PSCs share many functionalities. OSCs and PSCs fabrication involves sequential deposition of layers from solution and treatments at elevated temperatures, so the CTLs must be solvent resistant and stable under thermal cycling (also important for long-term device stability). Preferably, the CTLs should also have sufficient tolerance to thickness variations to facilitate roll-to-roll fabrication. The CTL requirements however differ for OSCs and PSCs due to the unique respective properties of the organic semiconductor and metal halide perovskite PALs, so we will discuss them separately below.
1. Perovskite semiconductor solar cells
A PSC is generally composed of several stacked functional layers, where the perovskite PAL is sandwiched between the n-type and p-type CTL, accomplished with metallic contacts as cathode and anode, respectively (Fig. 1(a)). The perovskite surface and created interfaces play critical roles in device efficiency and stability due to their rich chemical and electronic characters. Besides numerous surface defects that can trap and even annihilate photogenerated carriers, the electronic structure of the perovskite surface control the interface energy level alignment when perovskite contacts with CTL, and thus affects the charge transport, accumulation and recombination dynamics in PSCs. Unmatched interface energetics will form an injection barrier or extraction barrier for charge carriers depending on the energy-level offset, causing severe thermionic loss or charge recombination loss, respectively, governing the device output especially in terms of open-circuit voltage (Voc) and fill factor (FF) deficit[1, 2]. Moreover, the unmatched energetics induced charge accumulation can lead to hysteresis behavior and stability limit[3].
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To assess the interfacial energy level alignment and judge the interface from good or bad one, relevant energetic parameters such as work function (WF), valence band maximum (VBM, or highest occupied molecular orbital, HOMO for organic CTL materials) and conduction band minimum (CBM, or lowest unoccupied molecular orbital, LUMO for organic CTL materials) relative to Fermi level (EF) from both sides of the interface are in demand. Experimentally, the most commonly used technique to derive the energetic diagrams of the perovskite surface and its interface is photoelectron spectroscopy (PES). The PES-based toolbox can give direct information on the energy and density of occupied and unoccupied electronic states by ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES), respectively.
The intricacy of the interface energetics arises from the perovskite PAL, of which the electronic structure is dictated by several factors, such as precursor composition, fabrication process and substrate character. Perovskites are commonly formulated of ABX3 with a three-dimensional (3D) lattice structure, where A, B, and X refer to monovalent cation, divalent cation, and halide anion, respectively. Composition engineering by partial or complete substitution at A-site, B-site, or X-site has shown great impact on the electronic structure of perovskite semiconductors, rendering a unique set of tunable optoelectronic properties. For perovskites with specific composition, e.g., FAPbI3 or Cs0.05(FA0.95MA0.05)0.95Pb(I0.95Br0.05)3 widely employed in state-of-the-art PSCs, their energetics feature high dependence on the conductivity type and WF of the underlying electrode. We have established that the WF of the perovskite PAL increases with increasing WF of the underlying electrode so that the perovskite surface can undergo a clear transformation from n- (p-) to p- (n-) type semiconducting character (Fig. 1(b))[4−6]. The underlying electrode dictates the perovskite surface energetics in a remote doping way, where electrons transfer from the low WF electrode (perovskite) to the high WF perovskite (substrate) to achieve electrochemical potential equilibrium, attributed to the high defect tolerance and low deep-defect densities of perovskite semiconductor that provide sufficient depletion region for the swing of EF level in the bandgap[7]. Notably, the precise origin of this remote doping effect is still unclear, and more exploration on the perovskite heterointerface is highly needed.
Accordingly, the underlying electrode dependent perovskite surface energetics will form inconsistent or opposite energetic type relative to the top electron-transport layer (ETL) or hole-transport layer (HTL), which largely confines the carrier extraction efficacy and increases the probability of electron−hole recombination at the top heterointerface[8]. For n−i−p (p−i−n) structural PSC, a perovskite film grown on cathode (anode) with low (high) WF preferentially presents a more n- (p-) type surface energetic due to the self-doping, which is detrimental for the electrical contact with sequentially upper deposited p-type HTL (n-type ETL) and thus impede hole (electron) extraction across the created heterointerface from the perovskite PAL to the adjacent CTL, resulting in serious interface recombination in devices. This might be the origin that the top heterointerfaces (perovskite/ETL for p−i−n PSC and perovskite/HTL for n−i−p PSC) have been considered as the main source for nonradiative recombination[1].
Modulating perovskite surface energetics is thereby one key to suppress nonradiative recombination and improve device performance, which, in fact, has achieved great success in terms of both efficiency and stability[9, 10]. A number of powerful approaches including charge transfer doping, surface dipole formation, and surface reconstruction are employed to tune the interface properties and energy level alignment between perovskite and CTL, improving charge transport in perovskite-based optoelectronics[11−13]. Perovskite surfaces with lower (higher) WF is reported to reduce energetic mismatch and construct efficient charge selective contact with top ETL (HTL), prompting charge extraction and reducing charge recombination at the interface[14, 15]. It’s promising and challenging to construct perovskite p−n (n−p) homojunction via complete transformation of perovskite surface energetics from p- to n-type for p−i−n PSC and from n- to p-type for n−i−p PSC, respectively (Fig. 1(c)), which can provide extra build-in electric filed, and accelerate photogenerated electrons and holes moving to opposite directions[16, 17]. In addition, reconstructing the 3D perovskite surface into a lower dimensionality (2D) can also form aligned or graded band alignments with the top CTL, usually a HTL due to the p-type nature of 2D perovskite, enabling the integration of high efficiency with robust stability[18, 19]. Further development of 2D/3D heterojunction construction that favors electron extraction for highly efficient and stable p−i−n PSCs is thus timely and needed[20].
To unlock the full potential of PSCs, the following aspects should be considered from the viewpoint of interface energetics. It is always necessary to construct a PAL/CTL heterointerface with a negligible extraction barrier and minimum injection barrier for majority carriers, as well as sufficient blocking barrier for minority carriers. However, the mysteries of perovskite surface energetics are still uncovered, which requires feasible and non-destructive methods to explore the electronic structures down into the perovskite bulk and buried interface. Then, an energetic landscape of complete devices under working state (i.e., electric field, light illumination) will allow for disentangling the impacts of the perovskite surface and interface on charge transport behavior in the device. Moreover, it is highly demanded to explore the modulation mechanism of perovskite surface energetics via molecule dopants or functional layers, providing insights for novel energetic modifying strategy. Furthermore, it will be highly desirable to develop surface energetics modulators that possess the functionalities of reducing defects and enhancing stability, as well as the advances of low cost and large-area processing compatibility, promoting the commercialization of PSCs.
2. Organic semiconductor solar cells
The general device structure of an OSC is like that of PSCs, but the PAL here consist of donor- and acceptor-type organic semiconductors typically deposited from a mixed solution to form a so-called bulk heterojunction (BHJ) structure (Fig. 2(a)). Alternatively, to better control the vertical phase separation of the donor and acceptor components, sequential layer-by-layer (LbL) deposition of donor and acceptor layers can be carried out to form the PAL. The more homogenous distributed donor:acceptor mixture of the BHJ-type PALs is considered advantageous in terms of free-charge generation, whereas the significant vertical concentration gradient of LbL PALs facilitates improved charge transport and extraction[21]. Regardless if a BHJ or LbL PAL is chosen, the CTLs play several roles to enhance device performance and stability. Most obviously, the CTLs should just as in the PSC case promote barrier-less charge extraction of the majority carriers while blocking the minority carriers as well as the excitons from reaching the outer metallic electrode. In terms of the OSC fabrication, the CTL surface energy can tune the morphology of the PAL not only at the interface but also affecting the vertical concentration gradient of the PAL layer[22]. The surface of the organic semiconductor PAL does not suffer from charge trapping and exciton quenching defects as the perovskite PAL, but the CTLs must be chemically compatible with the PAL to avoid adverse reactions that can induce defects[23]. Furthermore, the CTLs should also prevent contact between the PAL and the outer metallic electrodes, preventing both adverse chemical interactions and decreasing the electrostatic coupling between the charge carriers (and excitons) with the image charges of the metallic contacts[24, 25]. Finally, the CTLs should be conducting enough to facilitate efficient charge transport at CTL thicknesses compatible with coating/printing, while not so highly conducting as to impair the charge selectivity of the CTL[26], or be capable of self-assembly into layers thin enough to enable efficient tunneling from the PAL into the metallic electrode[27].
Returning to the primary function of the CTLs in OSC, i.e., promoting barrier-less charge extraction of the majority carriers, we will take a deeper look at the electronic structure at the PAL/CTL and CTL/electrode interfaces. The same experimental techniques used for PSCs discussed earlier are also useful for probing the interface chemistry and energetics in OSCs. As for PSCs, mismatched interface energetics in OSCs will negatively affect device output both in terms of Voc and FF[21]. Taking the singly occupied acceptor LUMO and donor HOMO levels to represent the quasi-Fermi levels for the electron (EF,e) and hole (EF,h) charge carriers in the PAL, we see that they should be aligned to the cathode and anode EF, respectively. This will in almost all cases require that the anode-side CTL up-shifts the WF of the anode to align the anode EF with the PAL EF,h and that the cathode-side CTL down-shifts the WF of the cathode to align the cathode EF with the PAL EF,e (Fig. 2(b)). The mechanisms used for WF modulation are varied and material dependent[25], illustrated below with some commonly used CTLs.
(ⅰ) Conducting polymers and metal oxides nanoparticles can be solution-processed as inks with e.g. water and/or alcohol as solvents, offer tunable conductivity and WF through doping and provide good thickness variation tolerance. These CTLs shift the WF of the metallic electrodes through Fermi level equilibration driven charge transfer at the metal/CTL interface. Examples are PEDOT:PSS (p-type)[21], BBL:PCAT-K (n-type)[28], ZnO and SnO2 nanoparticles (n-type)[29]. Drawbacks include decreased charge selectivity due to the high conductivity and the metal oxide nanoparticle CTLs can suffer from additional problems such as reactive surface defects and photocatalytic response.
(ⅱ) Organic semiconducting polymers and molecules functionalized with amine or ionic pendant groups that enable solution in alcohols can offer improved charge selectivity at the cost of thickness tolerance. The WF modulation of such CTLs can be quite complex involving amine/ion electrostatic interaction with the metal electrode image charge causing a so-called double dipole induced WF shift at the interface[30] and can also involve ground state charge transfer between the organic semiconductor and the metal that further contributes to an interface WF shift[25, 31]. Self doping effects, i.e. doping of the conjugated backbone involving amine/ionic pendant groups or ionic groups embedded in the polymer backbone (electroactive ionenes) can improve the thickness tolerance[21], though again with a possible impairment of the charge selectivity. Examples of this class of CTLs are PDIN and its derivatives (amine-pendant ETLs), PCP-X (conjugated polyelectrolyte HTLs)[32], and NDI-DABC (electroactive ionene ETL)[33].
(ⅲ) Solution-processed self-assembled monolayers (SAMs) of tunneling thickness involving molecules or polymers can modulate the electrode WF at the interface by through-bond charge transfer at the bonding sites, by intrinsic molecular dipole moments that are uniformly aligned by the substrate-SAM bonding interaction and/or by the double-dipole effect[25]. Examples are carbozole phosphonic acids (HTL)[34] and polyvinylpyrrolidone (ETL)[35].
The outlook for the development of CTLs for OSCs is promising. New stable water-processable conducting polymer systems are under development[36] and the use of block polymerization as demonstrated by e.g. ionenes offer existing possibilities for adding functionality and fine-tuning thereof. A few possibly underexplored aspects of CTL development are the PAL−CTL interface energetics, as more efforts have been placed on understanding the WF modulation at the CTL/electrode contacts, and the use of more complex binary of ternary CTL systems to achieve the desired complex multi-functionality required for combined high efficiency, high stability and ease of processing.
Acknowledgements
The work is financially supported by the National Science Foundation of China grant (62322407, 22279034, 52261145698, W2421103), and Shanghai Science and Technology Innovation Action Plan (22ZR1418900, 24110714100). Linköping University thanks the Swedish Research Council (project grant no. 2020-04538) and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO Mat LiU no. 2009 00971).
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