ZiQi Sun

Organic/polymer semiconductors provide unique possibilities and flexibility in tailoring their optoelectronic properties to match specific application demands. One of the key factors contributing to the rapid and continuous progress of organic photovoltaics (OPVs) is the control and optimization of photoactive-layer morphology. The impact of morphology on photovoltaic parameters has been widely observed. However, the highly complex and multilength-scale morphology often formed in efficient OPV devices consisting of compositionally similar components impose obstacles to conventional morphological characterizations. In contrast, due to the high compositional and orientational sensitivity, resonant soft X-ray scattering (R-SoXS), and related techniques lead to tremendous progress of characterization and comprehension regarding the complex mesoscale morphology in OPVs. R-SoXS is capable of quantifying the domain characteristics, and polarized soft X-ray scattering (P-SoXS) provides quantitative information on orientational ordering. These morphological parameters strongly correlate the fill factor (FF), open-circuit voltage (V_oc), as well as short-circuit current (J_sc) in a wider range of OPV devices, including recent record-efficiency polymer:fullerene solar cells and 12%-efficiency fullerene-free OPVs. This progress report will delineate the soft X-ray scattering methodology and its future challenges to characterize and understand functional organic materials and provide a non-exhaustive overview of R-SoXS characterization and its implication to date.

Control and optimization of photoactive-layer morphology is a key strategy to boost the device performance of organic photovoltaics (OPVs). Quantitative morphological parameters (domain characteristics and orientational ordering) resolved from resonant and polarized soft X-ray scattering techniques are introduced and correlated well with the photovoltaic parameters in a wider range of OPV systems, including the record-efficiency polymer:fullerene and nonfullerene systems.

The mixed perovskite (FAPbI₃)_1−x(MAPbBr₃)_x, prepared by directly mixing different perovskite components, suffers from phase competition and a low-crystallinity character, resulting in instability, despite the high efficiency. In this study, a dual ion exchange (DIE) method is developed by treating as-prepared FAPbI₃ with methylammonium brodide (MABr)/tert-butanol solution. The converted perovskite thin film shows an optimized absorption edge at 800 nm after reaction time control, and the high crystallinity can be preserved after MABr incorporation. More importantly, it is found that the threshold electrical field to initiate ion migration is greatly increased in DIE perovskite thin film because excess MABr on the surface can effectively heal structural defects located on grain boundaries during the ion exchange process. It contributes to the over-one-month moisture stability under ≈65% room humidity (RH) and greatly enhanced light stability for the bare perovskite film. As a result of preserved high crystallinity and simultaneous grain boundary passivation, the perovskite solar cells fabricated by the DIE method demonstrate reliable reproducibility with an average power conversion efficiency (PCE) of 17% and a maximum PCE of 18.1%, with negligible hysteresis.

A dual ion exchange (DIE) method is developed for mixed perovskite thin films by treating trigonal FAPbI₃ with MABr in tert-butanol. This DIE method can preserve the initial high crystallinity and passivate vacancies/defects at grain boundaries, leading to enhanced moisture and illumination stability and reduced ion migration. The solar cell device using the DIE method achieves the highest power conversion efficiency of 18.1%, with negligible hysteresis.

Efficient wide-bandgap (WBG) perovskite solar cells are needed to boost the efficiency of silicon solar cells to beyond Schottky–Queisser limit, but they suffer from a larger open circuit voltage (V_OC) deficit than narrower bandgap ones. Here, it is shown that one major limitation of V_OC in WBG perovskite solar cells comes from the nonmatched energy levels of charge transport layers. Indene-C60 bisadduct (ICBA) with higher-lying lowest-unoccupied-molecular-orbital is needed for WBG perovskite solar cells, while its energy-disorder needs to be minimized before a larger V_OC can be observed. A simple method is applied to reduce the energy disorder by isolating isomer ICBA-tran3 from the as-synthesized ICBA-mixture. WBG perovskite solar cells with ICBA-tran3 show enhanced V_OC by 60 mV, reduced V_OC deficit of 0.5 V, and then a record stabilized power conversion efficiency of 18.5%. This work points out the importance of matching the charge transport layers in perovskite solar cells when the perovskites have a different composition and energy levels.

One major limitation of open-circuit voltage (V_OC) in wide-bandgap (WBG) perovskite solar cells comes from the nonmatched charge extraction contact. WBG perovskite solar cells with indene-C60 bisadduct-tran3 isomer with higher-lying lowest-unoccupied-molecular-orbital and reduced energy disorder show enhanced V_OC , and then a record stabilized power conversion efficiency of 18.5%.

High-performance ternary organic solar cells are fabricated by using a wide-bandgap polymer donor (bithienyl-benzodithiophene-alt-fluorobenzotriazole copolymer, J52) and two well-miscible nonfullerene acceptors, methyl-modified nonfullerene acceptor (IT-M) and 2,2′-((2Z,2′Z)-((5,5′-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydros-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IEICO). The two acceptors with complementary absorption spectra and similar lowest unoccupied molecular orbital levels show excellent compatibility in the blend due to their very similar chemical structures. Consequently, the obtained ternary organic solar cells (OSC) exhibits a high efficiency of 11.1%, with an enhanced short-circuit current density of 19.7 mA cm⁻² and a fill factor of 0.668. In this ternary system, broadened absorption, similar output voltages, and compatible morphology are achieved simultaneously, demonstrating a promising strategy to further improve the performance of ternary OSCs.

Ternary organic solar cells show over 11% power conversion efficiency by using two compatible nonfullerene acceptors with complementary absorption spectra, similar chemical structures, and similar lowest unoccupied molecular orbital levels. Broadened absorption, similar output voltages, and compatible morphology are achieved simultaneously, demonstrating a promising strategy to improve the performance of OSCs.

Tandem organic solar cells (TOSCs), which integrate multiple organic photovoltaic layers with complementary absorption in series, have been proved to be a strong contender in organic photovoltaic depending on their advantages in harvesting a greater part of the solar spectrum and more efficient photon utilization than traditional single-junction organic solar cells. However, simultaneously improving open circuit voltage (V_oc) and short current density (J_sc) is a still particularly tricky issue for highly efficient TOSCs. In this work, by employing the low-bandgap nonfullerene acceptor, IEICO, into the rear cell to extend absorption, and meanwhile introducing PBDD4T-2F into the front cell for improving V_oc, an impressive efficiency of 12.8% has been achieved in well-designed TOSC. This result is also one of the highest efficiencies reported in state-of-the-art organic solar cells.

Simultaneously improving the open-circuit voltage (V_oc) and short current density (J_sc) is a particularly tricky issue for tandem organic solar cells (TOSCs). By employing the low-bandgap nonfullerene acceptor, IEICO, in the rear cell to extend absorption, and meanwhile introducing PBDD4T-2F into the front cell for improving V_oc, an impressive efficiency of 12.8% is achieved in TOSCs. This result is also one of the highest efficiencies reported in state-of-the-art organic solar cells.

The ease of processing hybrid organic–inorganic perovskite (HOIPs) films, belonging to a material class with composition ABX₃, from solution and at mild temperatures promises their use in deformable technologies, including flexible photovoltaic devices, sensors, and displays. To successfully apply these materials in deformable devices, knowledge of their mechanical response to dynamic strain is necessary. The authors elucidate the time- and rate-dependent mechanical properties of HOIPs and an inorganic perovskite (IP) single crystal by measuring nanoindentation creep and stress relaxation. The observation of pop-in events and slip bands on the surface of the indented crystals demonstrate dislocation-mediated plastic deformation. The magnitudes of creep and relaxation of both HOIPs and IPs are similar, negating prior hypothesis that the presence of organic A-site cations alters the mechanical response of these materials. Moreover, these samples exhibit a pronounced increase in creep, and stress relaxation as a function of indentation rate whose magnitudes reflect differences in the rates of nucleation and propagation of dislocations within the crystal structures of HOIPs and IP. This contribution provides understanding that is critical for designing perovskite devices capable of withstanding mechanical deformations.

Dynamic mechanical response of hybrid organic–inorganic and inorganic perovskite crystals suggests that the time-dependent mechanical properties of lead–halide perovskites are independent of the chemical character of the A-site cation. Moreover, significant viscoplastic behavior is revealed through creep and stress-relaxation measurements. These phenomena are interpreted as direct results of the crystal structures and how dislocations propagate within them.

Colloidal quantum dots (CQDs) are promising light harvesting materials for realization of solution processible, highly efficient multipurpose photovoltaics (PVs). Here, PbS CQD solar cells are reported with improved certified power conversion efficiency performance of 10.4% by simply controlling protic solvents (alcohols) in ligand exchange process. With shorter chain alcohols, the mobility of charge carriers is an order-of-magnitude improved due to the enhanced interparticle coupling; on the other hand, excessive removal of passivating ligands by very protic solvent, methanol (MeOH) induced undesirable traps on CQD surface. Consequently, it has been found that high performance CQD PVs require a solvent engineering for balance between native leaving ligands with incoming ligands during ligand exchange process for well-controlled surfaces of CQDs and enhanced carrier concentration of conductive CQD films.

The control of short-chain alcohols in ligand exchange is proven to be very crucial for improving optoelectronic properties of PbS colloidal quantum dot (CQD) films. MeOH commonly used for ligand exchange of CQDs creates too many uncontrolled surface traps, but EtOH balances the ligand exchange and surface trap density, enabling a high certified power-conversion-efficiency of 10.4%.

Perylene diimide (PDI) with high electron affinities are promising candidates for applications in polymer solar cells (PSCs). In addition, the strength of π-deficient backbones and end-groups in an n-type self-dopable system strongly affects the formed end-group-induced electronic interactions. Herein, a series of amine/ammonium functionalized PDIs with excellent alcohol solubility are synthesized and employed as electron transporting layers (ETLs) in PSCs. The electron transfer properties of the resulting PDIs are dramatically tuned by different end-groups and π-deficient backbones. Notably, electron transfer is observed directly in solution in self-doped PDIs for the first time. A significantly enhanced power conversion efficiency of 10.06% is achieved, when applying the PDIs as ETLs in PTB7-Th:PC₇₁BM-based PSCs. These results demonstrate the potential of n-type organic semiconductors with stable n-type doping capability and facile solution processibility for future applications of energy transition devices.

Serials self-doped perylene diimides (PDIs) are investigated and applied as electron transporting layer in high-efficiency polymer solar cells. Variations of both the π-deficient PDI cores and the electron-donating amine/ammonium end-groups induce electronic interactions between them in different intensity, which facilitates the management of the electron transfer properties for applications in organic electronics.

Recently, organometal halide perovskite (OMHP)-based solar cells have been regarded as one of the most promising technologies in the research field of renewable energy applications. Along with successful demonstrations of high power conversion efficiencies (PCEs), various characteristic strategies for fabricating functional OMHP-based solar cells have been exploited to facilitate both their practical applicability and industrial suitability. As a part of such efforts, unconventional transparent conductive electrodes have been suggested based on the implementation of metal nanowires (MeNWs), which possess both high transparency and low sheet resistance, in order to replace traditional counterparts such as costly, limitedly-flexible vacuum-deposited conductive metal oxides. This allows for the facile fabrication of solution-processable, low-cost, highly flexible, high-performance solar cell devices. In this review, the recent progress on OMHP solar cells integrated with MeNW-network electrodes is investigated and the challenges associated with the integration of MeNW-network electrodes are comprehensively addressed with the suggestion of possible solutions for resolving the critical issues.

Integration between metal nanowire network-based electrodes and perovskite solar cells enables diversification of fabrication processes and functionalities of perovskite solar cells. High-performance, semi-transparent, and flexible perovskite solar cells with metal nanowires are expected to be fabricated without resorting to vacuum processes. The challenging issues facing the integration are also investigated.

In the most efficient solar cells based on blends of a conjugated polymer (electron donor) and a fullerene derivative (electron acceptor),ultrafast formation of charge-transfer (CT) electronic states at the donor-acceptor interfaces and efficient separation of these CT states into free charges, lead to internal quantum efficiencies near 100%. However, there occur substantial energy losses due to the non-radiative recombinations of the charges, mediated by the loweset-energy (singlet and triplet) CT states; for example, such recombinations can lead to the formation of triplet excited electronic states on the polymer chains, which do not generate free charges. This issue remains a major factor limiting the power conversion efficiencies (PCE) of these devices. The recombination rates are, however, difficult to quantify experimentally. To shed light on these issues, here, an integrated multi-scale theoretical approach that combines molecular dynamics simulations with quantum chemistry calculations is employed in order to establish the relationships among chemical structures, molecular packing, and non-radiative recombination losses mediated by the lowest-energy charge-transfer states.

In many polymer–fullerene bulk heterojunction solar cells, triplet exciton formation on the polymer chains has been identified as a major energy loss channel. Here, an integrated multiscale theoretical approach is used to establish detailed relationships among chemical structures, molecular packing, and nonradiative recombination loss mediated by the lowest-energy charge-transfer states with either singlet or triplet spin character.

One advantage of nonfullerene polymer solar cells (PSCs) is that they can yield high open-circuit voltage (V_OC) despite their relatively low optical bandgaps. To maximize the V_OC of PSCs, it is important to fine-tune the energy level offset between the donor and acceptor materials, but in a way not negatively affecting the morphology of the donor:acceptor (D:A) blends. Here, an effective material design rationale based on a family of D–A1–D–A2 terthiophene (T3) donor polymers is reported, which allows for the effective tuning of energy levels but without any negative impacts on the morphology of the blend. Based on a T3 donor unit combined with difluorobenzothiadiazole (ffBT) and difluorobenzoxadiazole (ffBX) acceptor units, three donor polymers are developed with highly similar morphological properties. This is particularly surprising considering that the corresponding quaterthiophene polymers based on ffBT and ffBX exhibit dramatic differences in their solubility and morphological properties. With the fine-tuning of energy levels, the T3 polymers yield nonfullerene PSCs with a high efficiency of 9.0% for one case and with a remarkably low energy loss (0.53 V) for another polymer. This work will facilitate the development of efficient nonfullerene PSCs with optimal energy levels and favorable morphology properties.

A terthiophene-based donor polymer PffBTBX-T3 with the D–A1–D–A2 structure is demonstrated and compared with the other two D–A-type polymers PffBT-T3 and PffBX-T3. While the energy levels of three polymers are fine-tuned, their optical and morphological pro­perties are not dramatically changed. The optimal energetic match and well-controlled morphology enable 9.0% effi­cient nonfullerene devices based on PffBTBX-T3 and ITIC-Th.

In article number 1605988, Chul-Ho Lee, Min Jae Ko, and co-workers report the structural engineering of formamidinium lead iodide (FAPbI₃) perovskite thin films by partially substituting the formamidinium cations with smaller rubidium (Rb) cations. Even traces of Rb significantly enhance photovoltaic performances and long-term stability of perovskite solar cells. This is due to the supplement favoring complete conversion of the perovskite to its photoactive phase while offering structural stabilization.