ZiQi Sun

For the first time, the plasmonic gold bipyramids (Au BPs) are introduced to the PbS colloidal quantum dot (CQD) solar cells for improved infrared light harvesting. The localized surface plasmon resonance peaks of Au BPs matches perfectly with the absorption peaks of conventional PbS CQDs. Owing to the geometrical novelty of Au BPs, they exhibit significantly stronger far-field scattering effect and near-field enhancement than conventional plasmonic Au nanospheres (NSs). Consequently, device open-circuit voltage (V_oc) and short-circuit current (J_sc) are simultaneously enhanced, while plasmonic photovoltaic devices based on Au NSs only achieve improved J_sc. The different effects and working mechanisms of these two Au nanoparticles are systematically investigated. Moreover, to realize effective broadband light harvesting, Au BPs and Au NSs are used together to simultaneously enhance the device optical and electrical properties. As a result, a significantly increased power conversion efficiency (PCE) of 9.58% is obtained compared to the PCE of 8.09% for the control devices due to the synergistic effect of the two plasmonic Au nanoparticles. Thus, this work reveals the intriguing plasmonic effect of Au BPs in CQD solar cells and may provide insight into the future plasmonic enhancement for solution-processed new-generation solar cells.

For the first time, the plasmonic gold bipyramids (Au BPs) are introduced to the PbS colloidal quantum dots solar cells. To realize broadband light harvesting, Au BPs and Au nanospheres (NSs) are used together. As a result, a significantly increased power conversion efficiency (PCE) of 9.58% is obtained due to the synergistic effect of the two plasmonic Au nanoparticles.

The future commercialization of halide perovskite solar cells relies on improving their stability. There are several studies focused on understanding degradation under operating conditions in light, but little is known about the stability of these solar cells under reverse bias conditions. Reverse bias stability is important because shaded cells in a module are put into reverse bias by the illuminated cells. In this paper, a phenomenological study is presented of the reverse bias behavior of halide perovskite solar cells and it is shown that reverse bias can lead to a partially recoverable loss in efficiency, primarily caused by a decrease in V _OC. A general mechanism is proposed, supported by drift–diffusion simulations, to explain how these cells breakdown via tunneling caused by accumulated ionic defects and suggests that the reversible loss in efficiency may be due to an electrochemical reaction of these defects. Finally, the implications of these phenomena are discussed and how they can possibly be addressed is also discussed.

The stability of halide perovskite solar cells in reverse bias is investigated. The cells uniformly pass current across the device at breakdown voltages between –1 and −4 V. A partially recoverable decrease in open-circuit voltage is seen for cells held in reverse bias. Drift–diffusion modeling supports breakdown via tunneling, and the implications and some possible solutions are discussed.

Side-chain engineering is an important strategy for optimizing photovoltaic properties of organic photovoltaic materials. In this work, the effect of alkylsilyl side-chain structure on the photovoltaic properties of medium bandgap conjugated polymer donors is studied by synthesizing four new polymers J70, J72, J73, and J74 on the basis of highly efficient polymer donor J71 by changing alkyl substituents of the alkylsilyl side chains of the polymers. And the photovoltaic properties of the five polymers are studied by fabricating polymer solar cells (PSCs) with the polymers as donor and an n-type organic semiconductor (n-OS) m-ITIC as acceptor. It is found that the shorter and linear alkylsilyl side chain could afford ordered molecular packing, stronger absorption coefficient, higher charge carrier mobility, thus results in higher J_sc and fill factor values in the corresponding PSCs. While the polymers with longer or branched alkyl substituents in the trialkylsilyl group show lower-lying highest occupied molecular orbital energy levels which leads to higher V_oc of the PSCs. The PSCs based on J70:m-ITIC and J71:m-ITIC achieve power conversion efficiency (PCE) of 11.62 and 12.05%, respectively, which are among the top values of the PSCs reported in the literatures so far.

Side-chain engineering is performed to optimize photovoltaic properties of the 2D-conjugated polymer donors. The polymer solar cells with m-ITIC as acceptor and J70 and J71 polymer donors with shorter and linear alkyl substituents in their alkylsilyl side chains achieve power conversion efficiency of 11.62% and 12.05%, respectively.

A modified detailed balance model is built to understand and quantify efficiency loss of perovskite solar cells. The modified model captures the light-absorption-dependent short-circuit current, contact and transport-layer-modified carrier transport, as well as recombination and photon-recycling-influenced open-circuit voltage. The theoretical and experimental results show that for experimentally optimized perovskite solar cells with the power conversion efficiency of 19%, optical loss of 25%, nonradiative recombination loss of 35%, and ohmic loss of 35% are the three dominant loss factors for approaching the 31% efficiency limit of perovskite solar cells. It is also found that the optical loss climbs up to 40% for a thin-active-layer design. Moreover, a misconfigured transport layer introduces above 15% of energy loss. Finally, the perovskite-interface-induced surface recombination, ohmic loss, and current leakage should be further reduced to upgrade device efficiency and eliminate hysteresis effect. This work contributes to fundamental understanding of device physics of perovskite solar cells. The developed model offers a systematic design and analysis tool to photovoltaic science and technology.

A modified detailed balance model is built to understand and quantify the efficiency loss of perovskite solar cells. The optical loss, nonradiative recombination loss, and ohmic loss are identified quantitatively. The perovskite-interface-induced surface recombination, ohmic loss, and current leakage are also analyzed.

Based on the most recently significant progress within the last one year in organic photovoltaic research from either alkylthiolation or fluorination on benzo[1,2-b:4,5-b′]dithiophene moiety for high efficiency polymer solar cells (PSCs), two novel simultaneously fluorinated and alkylthiolated benzo[1,2-b:4,5-b′] dithiophene (BDT)-based donor–acceptor (D–A) polymers, poly(4,8-bis(5′-((2″-ethylhexyl)thio)-4′-fluorothiophen-2′-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-alt-2′-ethylhexyl-3-fluorothieno[3,4-b]thiophene-2-carboxylate (PBDTT-SF-TT) and poly(4,8-bis(5′-((2″-ethylhexyl)thio)-4′-fluorothiophen-2′-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-alt-1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (PBDTT-SF-BDD), namely, via an advantageous and synthetically economic route for the key monomer are reported herein. Synergistic effects of fluorination and alkylthiolation on BDT moieties are discussed in detail, which is based on the superior balance between high V_oc and large J_sc when PBDTT-SF-TT/PC₇₁BM and PBDTT-SF-BDD/PC₇₁BM solar cells present their high V_oc as 1.00 and 0.97 V (associated with their deep highest occupied molecular orbital level of −5.54 and −5.61 eV), a moderately high J_sc of 14.79 and 14.70 mA cm⁻², and thus result a high power conversion efficiency of 9.07% and 9.72%, respectively. Meanwhile, for PBDTT-SF-TT, a very low energy loss of 0.59 eV is pronounced, leading to the promisingly high voltage, and furthermore performance study and morphological results declare an additive-free PSC from PBDTT-SF-TT, which is beneficial to practical applications.

Superior balance between high V_oc and large J_sc is realized via synergistic effect of fluorination and alkylthiolation on benzo[1,2-b:4,5-b′] dithiophene (BDT) moiety, leading to new efficient conventional BDT-based polymer solar cells are achieved with high power conversion efficiency of 9.07% for poly(4,8-bis(5′-((2″-ethylhexyl)thio)-4′-fluorothiophen-2′-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-alt-2′-ethylhexyl-3-fluorothieno[3,4-b]thiophene-2-carboxylate and 9.72% for poly(4,8-bis(5′-((2″-ethylhexyl)thio)-4′-fluorothiophen-2′-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-alt-1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione.

Organic solar cell optimization requires careful balancing of current–voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor–donor–acceptor (A–D–A)-type small-molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near-IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the V_oc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT- and NFBDT-based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge-pair states is observed, followed by effective charge extraction. As a result, the PBDB-T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%—among the best for solution-processed organic solar cells.

An acceptor-donor-acceptor nonfullerene acceptor NCBDT is reported. NCBDT exhibits a low optical bandgap of 1.45 eV and broadened absorption range. The PBDB-T:NCBDT-based device achieves an impressive PCE of 12.12% and J_sc over 20 mA cm^-2—one of the best results for solution-processed OSCs. Further photophysical study reveals slow (≈400 ps) yet efficient free charge generation.

Because of the rapid rise of the efficiency, perovskite solar cells are currently considered as the most promising next-generation photovoltaic technology. Much effort has been made to improve the efficiency and stability of perovskite solar cells. Here, it is demonstrated that the addition of a novel organic cation of 2-(6-bromo-1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)ethan-1-ammonium iodide (2-NAM), which has strong Lewis acid and base interaction (between CO and Pb) with perovskite, can effectively increase crystalline grain size and reduce charge carrier recombination of the double cation FA_0.83MA_0.17PbI_2.51Br_0.49 perovskite film, thus boosting the efficiency from 17.1 ± 0.8% to 18.6 ± 0.9% for the 0.1 cm² cell and from 15.5 ± 0.5% to 16.5 ± 0.6% for the 1.0 cm² cell. The champion cell shows efficiencies of 20.0% and 17.6% with active areas of 0.1 and 1.0 cm², respectively. Moreover, the hysteresis behavior is suppressed and the stability is improved. The result provides a promising route to further elevate efficiency and stability of perovskite solar cells by the fine tuning of triple organic cations.

A new organic additive (2-NAM) is introduced into the perovskite film. The introduction of this additive boosts the efficiency from 17.1 ± 0.8% to 18.6 ± 0.9% for the 0.1 cm² area cells and from 15.5 ± 0.5% to 16.5 ± 0.6% for the 1.0 cm² area cells. Moreover, the hydrophobic nature of this additive effectively reduces the influence from moisture, thus enhancing the solar cell stability.

Two novel wide-bandgap copolymers, PBDT-TDZ and PBDTS-TDZ, are developed based on 1,3,4-thiadiazole (TDZ) and benzo[1,2-b:4,5-b′]dithiophene (BDT) building blocks. These copolymers exhibit wide bandgaps over 2.07 eV and low-lying highest occupied molecular orbital (HOMO) levels below −5.35 eV, which match well with the typical low-bandgap acceptor of ITIC, resulting in a good complementary absorption from 300 to 900 nm and a low HOMO level offset (≤0.13 eV). Compared to PBDT-TDZ, PBDTS-TDZ with alkylthio side chains exhibits the stronger optical absorption, lower-lying HOMO level, and higher crystallinity. By using a single green solvent of o-xylene, PBDTS-TDZ:ITIC devices exhibit a large open-circuit voltage (V_oc) up to 1.10 eV and an extremely low energy loss (E_loss) of 0.48 eV. At the same time, the desirable high short-circuit current density (J_sc) of 17.78 mA cm⁻² and fill factor of 65.4% are also obtained, giving rise to a high power conversion efficiency (PCE) of 12.80% without any additive and post-treatment. When adopting a homotandem device architecture, the PCE is further improved to 13.35% (certified as 13.19%) with a much larger V_oc of 2.13 V, which is the best value for any type of homotandem organic solar cells reported so far.

Two novel 1,3,4-thiadiazole-based wide-bandgap copolymers, PBDT-TDZ and PBDTS-TDZ, are developed for efficient nonfullerene organic solar cells. The single-junction devices processed by a green solvent of o-xylene exhibit a high power conversion efficiency (PCE) of 12.80% with a low energy loss of 0.48 eV. The PCE is finally improved to 13.35% when using a homotandem device architecture.

In article number 1701305, Maria Antonietta Loi and co-workers show that the electron transport capability of the electron-extracting layer dominates the temperature dependence of the performance of hybrid perovskite solar cells. The authors further demonstrate efficient hybrid perovskite solar cells over a temperature range from 295 K to 160 K by n-doping the PCBM electron-extracting layer or using a fullerene derivative with intrinsically higher electron transport capability.

Flexible and semitransparent organic solar cells (OSCs) have been regarded as the most promising photovoltaic devices for the application of OSCs in wearable energy resources and building-integrated photovoltaics. Therefore, the flexible and semitransparent OSCs have developed rapidly in recent years through the synergistic efforts in developing novel flexible bottom or top transparent electrodes, designing and synthesizing high performance photoactive layer and low temperature processed electrode buffer layer materials, and device architecture engineering. To date, the highest power conversion efficiencies have reached over 10% of the flexible OSCs and 7.7% with average visible transmittance of 37% for the semitransparent OSCs. Here, a comprehensive overview of recent research progresses and perspectives on the related materials and devices of the flexible and semitransparent OSCs is provided.

Flexible and semitransparent organic solar cells (OSCs) are regarded as the most promising photovoltaic devices for the application of OSCs in wearable energy resources and building-integrated photovoltaics. Here, a comprehensive overview of recent research progresses and perspectives on the related materials and devices of the flexible and semitransparent OSCs is provided.

Organic–inorganic perovskites are well suited for optoelectronic applications. In particular, perovskite single and perovskite tandem solar cells with silicon are close to their market entry. Despite their swift rise in efficiency to more than 21%, solar cell lifetimes are way below the needed 25 years. In fact, comparison of the time when the device performance has degraded to 80% of its initial value (T₈₀ lifetime) of numerous solar cells throughout the literature reveals a strongly reduced stability under illumination. Herein, the various detrimental effects are discussed. Most notably, moisture- and heat-related degradation can be mitigated easily by now. Recently, however, several photoinduced degradation mechanisms have been observed. Under illumination, mixed perovskites tend to phase segregate, while, further, oxygen catalyzes deprotonation of the organic cations. Additionally, during illumination photogenerated charge can be trapped in the NH antibonding orbitals causing dissociation of the organic cation. On the other hand, organic–inorganic perovskites exhibit a high radiation hardness that is superior to crystalline silicon. Here, the proposed degradation mechanisms reported in the literature are thoroughly reviewed and the microscopic mechanisms and their implications for solar cells are discussed.

T₈₀ lifetimes of organic–inorganic perovskite solar cells are strongly reduced under illumination. Various degradation mechanisms are therefore discussed throughout the literature. Degradation by moisture or heat is well understood and mitigation possible. Photoinduced phase segregation and photoinduced dissociation of the organic cation, however, remain unsolved. Recent observations enlighten the underlying microscopic mechanisms and may pave the way for stable perovskites.

Naphtho[1,2-b:5,6-b′]dithiophene is extended to a fused octacyclic building block, which is end capped by strong electron-withdrawing 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile to yield a fused-ring electron acceptor (IOIC2) for organic solar cells (OSCs). Relative to naphthalene-based IHIC2, naphthodithiophene-based IOIC2 with a larger π-conjugation and a stronger electron-donating core shows a higher lowest unoccupied molecular orbital energy level (IOIC2: −3.78 eV vs IHIC2: −3.86 eV), broader absorption with a smaller optical bandgap (IOIC2: 1.55 eV vs IHIC2: 1.66 eV), and a higher electron mobility (IOIC2: 1.0 × 10⁻³ cm² V⁻¹ s⁻¹ vs IHIC2: 5.0 × 10⁻⁴ cm² V⁻¹ s⁻¹). Thus, IOIC2-based OSCs show higher values in open-circuit voltage, short-circuit current density, fill factor, and thereby much higher power conversion efficiency (PCE) values than those of the IHIC2-based counterpart. In particular, as-cast OSCs based on FTAZ: IOIC2 yield PCEs of up to 11.2%, higher than that of the control devices based on FTAZ: IHIC2 (7.45%). Furthermore, by using 0.2% 1,8-diiodooctane as the processing additive, a PCE of 12.3% is achieved from the FTAZ:IOIC2-based devices, higher than that of the FTAZ:IHIC2-based devices (7.31%). These results indicate that incorporating extended conjugation into the electron-donating fused-ring units in nonfullerene acceptors is a promising strategy for designing high-performance electron acceptors.

A novel fused-ring electron acceptor (IOIC2) based on naphthodithiophene is designed and synthesized, and compared with a naphthalene-based counterpart (IHIC2). The IOIC2-based single-junction binary-blend organic solar cells exhibit efficiencies up to 12.3%, much higher than that of IHIC2 (7.45%).

A series of PBDB-TTn random donor copolymers is synthesized, consisting of an electron-deficient benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) unit and different ratios of two electron-rich benzo[1,2-b:4,5-b′]dithiophene (BDT) and thieno[3,2-b]thiophene (TT) units, with intention to modulate the intrachain and/or interchain interactions and ultimately bulk-heterojunction morphology evolution. A comparative study using 4 × 2 polymer solar cell (PSC) performance maps and each of the [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) and the fused-aromatic-ring-based molecule (m-ITIC) acceptors are carried out. Given the similarities in their absorption ranges and energy levels, the PBDB-TTn copolymers clearly reveal a change in the absorption coefficients upon optimization of the BDT to TT ratio in the backbone. Among the given acceptor combination sets, superior performances are observed in the case of PBDB-TT5 blended with PC₇₁BM (8.34 ± 0.10%) or m-ITIC (11.10 ± 0.08%), and the dominant factors causing power conversion efficiency differences in them are found to be distinctly different. For example, the performances of PC₇₁BM-based PSCs are governed by size and population of face-on crystallites, while intermixed morphology without the formation of large phase-separated aggregates is the key factor for achieving high-performance m-ITIC-based PSCs. This study presents a new sketch of structure–morphology–performance relationships for fullerene- versus nonfullerene-based PSCs.

BDD-based four copolymers PBDD-TTn which contained BDT, TT, and BDD are synthesized and operated with two acceptors, PC₇₁BM and m-ITIC. Two systems have different operating mechanisms, and simultaneously high-performances 8.44% for PC₇₁BM and 11.18% for m-ITIC are obtained.