Chen Weijie

Hybrid perovskites are on a trajectory toward realizing the most efficient single-junction, solution-processed photovoltaic devices. However, a critical issue is the limited understanding of the correlation between the degree of crystallinity and the emergent perovskite/hole (or electron) transport layer on device performance and photostability. Here, the controlled growth of hybrid perovskites on nickel oxide (NiO) is shown, resulting in the formation of thin films with enhanced crystallinity with characteristic peak width and splitting reminiscent of the tetragonal phase in single crystals. Photophysical and interface sensitive measurements reveal a reduced trap density at the perovskite/NiO interface in comparison with perovskites grown on poly(3,4-ethylene dioxy thiophene) polystyrene sulfonate. Photovoltaic cells exhibit a high open circuit voltage (1.12 V), indicating a near-ideal energy band alignment. Moreover, photostability of photovoltaic devices up to 10-Suns is observed, which is a direct result of the superior crystallinity of perovskite thin films on NiO. These results elucidate the critical role of the quality of the perovskite/hole transport layer interface in rendering high-performance and photostable optoelectronic devices.

Highly crystalline perovskite thin film can be grown on nickel oxide substrates evidenced by sharp X-ray diffraction pattern with characteristic tetragonal peak splitting observed only in single crystal. As a consequence, high-efficiency photovoltaic cells can be achieved with extended operation lifetime under constant illumination benefit by the high degree of crystallinity.

High-performance colored aesthetic semitransparent organic photovoltaics (OPVs) featuring a silver/indium tin oxide/silver (Ag/ITO/Ag) microcavity structure are prepared. By precisely controlling the thickness of the ITO layer, OPV devices exhibiting high transparency and a wide and high-purity color gamut are obtained: blue (B), green (G), yellow-green (YG), yellow (Y), orange (O), and red (R). The power conversion efficiencies (PCEs) of the G, YG, and Y color devices are greater than 8% (AM 1.5G irradiation, 100 mW cm⁻²) with maximum transmittances (T_MAX) of greater than 14.5%. An optimized PCE of 8.2% was obtained for the YG OPV [CIE 1931 coordinates: (0.364, 0.542)], with a value of T_MAX of 17.3% (at 561 nm). As far as it is known, this performance is the highest ever reported for a transparent colorful OPV. Such high transparency and desired transmitted colors, which can perspective see the clear images, suggest great potential for use in building-integrated photovoltaic applications.

High-performance colored aesthetic semitransparent organic photovoltaics (OPVs) featuring a silver/indium tin oxide/silver microcavity structure are demonstrated. Colored OPVs of high purity and a wide color gamut are obtained: blue, green, yellow-green, yellow, orange, and red. The highest power conversion efficiency was 8.2% for the yellow-green device, with CIE 1931 coordinates of (0.364, 0.542) and a transmittance of 17.3% at 561 nm.

Organic photovoltaic cells possess desirable practical characteristics, such as the potential for low-cost fabrication on flexible substrates, but they lag behind their inorganic counterparts in performance due in part to fundamental energy loss mechanisms, such as overcoming the charge transfer (CT) state binding energy when photogenerated charge is transferred across the donor/acceptor interface. However, recent work has suggested that crystalline interfaces can reduce this binding energy due to enhanced CT state delocalization. Solar cells based on rubrene and C₆₀ are investigated as an archetypal system because it allows the degree of crystallinity to be moldulated from a highly disordered to highly ordered system. Using a postdeposition annealing method to transform as-deposited amorphous rubrene thin films into ones that are highly crystalline, it is shown that the CT state of a highly crystalline rubrene/C₆₀ heterojunction undergoes extreme delocalization parallel to the interface leading to a band-like state that exhibits a linear Stark effect. This state parallels the direct charge formation of inorganic solar cells and reduces energetic losses by 220 meV compared with 12 other archetypal heterojunctions reported in the literature.

Highly crystalline organic solar cells are investigated, and reduced voltage loss attributed to a band-like charge transfer state is found.

Triplet materials have been employed to achieve high-performing organic solar cells (OSCs) by extending the exciton lifetime and diffusion distances, while the triplet non-fullerene acceptor materials have never been reported for bulk heterojunction OSCs. Herein, for the first time, three triplet molecular acceptors based on tellurophene with different degrees of ring fusing were designed and synthesized for OSCs. Significantly, these molecules have long exciton lifetime and diffusion lengths, leading to efficient power conversion efficiency (7.52 %), which is the highest value for tellurophene-based OSCs. The influence of the extent of ring fusing on molecular geometry and OSCs performance was investigated to show the power conversion efficiencies (PCEs) continuously increased along with increasing the extent of ring fusing.

Solar fusion: Three triplet molecular acceptors based on tellurophene with different degrees of ring fusing were prepared for organic solar cells (OSCs). These molecules have long exciton lifetime and diffusion lengths, leading to increasing power conversion efficiency (PCE) up to 7.52 %, which is the highest value for tellurophene-based OSCs, as the degree of ring-fusion increases.

To maximize the short-circuit current density (J_SC) and the open circuit voltage (V_OC) simultaneously is a highly important but challenging issue in organic solar cells (OSCs). In this study, a benzotriazole-based p-type polymer (J61) and three benzotriazole-based nonfullerene small molecule acceptors (BTA1-3) are chosen to investigate the energetic driving force for the efficient charge transfer. The lowest unoccupied molecular orbital (LUMO) energy levels of small molecule acceptors can be fine-tuned by modifying the end-capping units, leading to high V_OC (1.15–1.30 V) of OSCs. Particularly, the LUMO energy level of BTA3 satisfies the criteria for efficient charge generation, which results in a high V_OC of 1.15 V, nearly 65% external quantum efficiency, and a high power conversion efficiency (PCE) of 8.25%. This is one of the highest V_OC in the high-performance OSCs reported to date. The results imply that it is promising to achieve both high J_SC and V_OC to realize high PCE with the carefully designed nonfullerene acceptors.

The existence of the driving force in organic solar cells (OSCs) often creates a problematic trade-off between the open-circuit voltage and short-circuit current. Here, fine-tuning driving force by gradually decreasing the acceptor energy level has afforded high open-circuit voltage (>1.15 V) and efficient charge generation (>60%) at the same time, which is instructive to the development of more efficient OSCs.

Despite the potential of ternary polymer solar cells (PSCs) to improve photocurrents, ternary architecture is not widely utilized for PSCs because its application has been shown to reduce fill factor (FF). In this paper, a novel technique is reported for achieving highly efficient ternary PSCs without this characteristic sharp decrease in FF by matching the highest occupied molecular orbital (HOMO) energy levels of two donor polymers. Our ternary device—made from a blend of wide-bandgap poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-alt-2,5-dioctyl-4,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione) (PBDT-DPPD) polymer, narrow-bandgap poly[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2- 6-diyl)] (PTB7-Th) polymer, and [6,6]-phenyl C₇₀-butyric acid methyl ester (PC₇₀BM)—exhibits a maximum power conversion efficiency of 10.42% with an open-circuit voltage of 0.80 V, a short-circuit current of 17.61 mA cm⁻², and an FF of 0.74. In addition, this concept is extended to quaternary PSCs made by using three different donor polymers with similar HOMO levels. Interestingly, the quaternary PSCs also yield a good FF (≈0.70)—similar to those of corresponding binary PSCs. This study confirms that the HOMO levels of the polymers used on the photoactive layer of PSCs are a crucial determinant of a high FF.

Highly efficient ternary polymer solar cells (PSCs) are successfully demonstrated by matching the highest occupied molecular orbital (HOMO) energy levels of two donor polymers. Ternary or quaternary PSCs made using two or more donor polymers with similar HOMO levels allow efficient charge transport, and consequently offer notably a higher fill factor and power conversion efficiency.

Highly efficient organic–inorganic hybrid solar cells of Si-poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) have been demonstrated by simultaneous structural, electrical, and interfacial engineering with low processing temperature. Si substrate has been sculpted into hierarchical structure to reduce light reflection loss and increase interfacial junction area at the same time. Regarding the electrical optimization, highly conductive organic PEDOT:PSS layer has been formulated with low sheet resistance. It is argued that the sheet resistance, rather than conductivity, is the primary parameter for the high efficiency hybrid cells, which leads to the optimization of thickness, i.e., thick enough to have low sheet resistance but transparent enough to pass the incident sunlight. Finally, siloxane oligomers have been inserted into top/bottom interfaces by contact-printing at room ambient, which suppresses carrier recombination at interfaces and reduces contact resistance at bottom electrode. Contrary to high-temperature doping (for the formation of front surface or back surface fields), wet solution processes or vacuum-based deposition, the contact-printing can be done at room ambient to reduce carrier recombination at the interfaces. The high efficiency obtained with low processing temperature can make this type of cells be a possible candidate for post-Si photovoltaics.

High (>17%) efficiency Si-PEDOT:PSS hybrid solar cells are demonstrated by simultaneous optimization of Si surface structure, electrical property of PEDOT:PSS, and interfacial passivation by contact-printed siloxane oligomers. Notably, the results are obtained without the adoption of vacuum-based or high-temperature-based processes. The hybrid cell may be the realistic candidate for post-Si photovoltaics.

Thin film solar cells based in Cu(In,Ga)Se₂ (CIGS) are among the most efficient polycrystalline solar cells, surpassing CdTe and even polycrystalline silicon solar cells. For further developments, the CIGS technology has to start incorporating different solar cell architectures and strategies that allow for very low interface recombination. In this work, ultrathin 350 nm CIGS solar cells with a rear interface passivation strategy are studied and characterized. The rear passivation is achieved using an Al₂O₃ nanopatterned point structure. Using the cell results, photoluminescence measurements, and detailed optical simulations based on the experimental results, it is shown that by including the nanopatterned point contact structure, the interface defect concentration lowers, which ultimately leads to an increase of solar cell electrical performance mostly by increase of the open circuit voltage. Gains to the short circuit current are distributed between an increased rear optical reflection and also due to electrical effects. The approach of mixing several techniques allows us to make a discussion considering the different passivation gains, which has not been done in detail in previous works. A solar cell with a nanopatterned rear contact and a 350 nm thick CIGS absorber provides an average power conversion efficiency close to 10%.

In this work ultrathin 350 nm Cu(In,Ga)Se₂ (CIGS) solar cells with passivated rear contact are studied and characterized. The rear interface passivation is achieved using an Al₂O₃ nanopatterned point structure and an average power conversion efficiency close to 10% is achieved due to chemical passivation and increased light trapping.

Nanostructures produce unique optical and electronic properties, which have the potential to meet the goals of third-generation photovoltaic devices. However, most nanostructures bring accompanying optical or electrical losses to solar cells. In article number 1602385, Jr-Hau He and Hsin-Ping Wang postulate that the concurrent design of both optical and electrical components will be an imperative route toward breaking the present-day limit of nanostructured solar cells.

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.