• Energy Research

ConspectusOrganic photovoltaics (OPVs) with a photoactive layer containing a blend of organic donor and acceptor species are considered to be a promising technology for clean energy owing to their unique flexible form factor and good solution processability that can potentially address the scalability challenges. The delicate designs of both donors and acceptors have significantly enhanced the power conversion efficiency of OPVs to more than 18%. Nonfullerene small-molecule acceptors (NFAs) have played a critical role in enhancing the short-circuit current density (JSC) by efficiently harvesting near-infrared (NIR) sunlight. To take full advantage of the abundant NIR photons, the optical band gap of NFAs should be further reduced to improve the performance of OPVs. Incorporating highly polarizable selenium atoms onto the backbone of organic conjugated materials has been proven to be an effective way to decrease their optical band gap. For example, a selenium-substituted NFA recently developed by our group has attained a JSC of approximate 27.5 mA cm-2 in OPV devices, surpassing those of most emerging photovoltaic systems. Inspired by this advance, we concentrate on the topic of selenium-containing materials in this Account to incite readers' interest in further exploring this series of materials.In this Account, we first compare the differences among chalcogen heterocycles and discuss the influence of fundamental electronic behavior on the collective photoelectrical properties of the resulting materials. The superior features of selenium-substituted materials are summarized as follows: (1) The large covalent radius of selenium can diminish the π-orbital overlap, rendering enhanced quinoidal resonance character and a narrowed optical band gap of resulting materials. (2) The selenium atom is more polarizable than sulfur owing to its larger and looser outermost electron cloud, enabling enhanced intermolecular Se-Se interaction and increased charge carrier mobility of relevant materials in the solid state. We then focus on summarizing the design rules for various categories of selenium-containing materials including polymer donors, small-molecule acceptors, and polymer acceptors, especially those composed of ladder-type polycyclic units. The motivation for incorporating selenium atoms into these materials and the structure-property relationships were thoroughly elucidated. Specifically, we discuss the changes in the optical band gap, charge carrier mobility, and molecular packing induced by selenium substitution and correlate the effects of these changes with the exciton behaviors, energy loss, and nanoscale film morphology of corresponding OPV devices. Furthermore, we point out the intrinsic stability of selenium-containing materials under maximum-power-point tracking and long-term photo- or thermostress and indicate their potential use in semitransparent and tandem solar cells. At the end, the prospect of future research focuses and the possible applications of selenium-containing materials in the OPV field are discussed.

The performance of organic photovoltaics is largely dependent on the balance of short-circuit current density (JSC) and open-circuit voltage (VOC). For instance, the reduction of the active materials’ optical bandgap, which increases the JSC, would inevitably lead to a concomitant reduction in VOC. Here, we demonstrate that careful tuning of the chemical structure of photoactive materials can enhance both JSC and VOC simultaneously. Non-fullerene organic photovoltaics based on a well-matched materials combination exhibit a certified high power conversion efficiency of 12.25% on a device area of 1 cm2. By combining Fourier-transform photocurrent spectroscopy and electroluminescence, we show the existence of a low but non-negligible charge transfer state as the possible origin of VOC loss. This study highlights that the reduction of the bandgap to improve the efficiency requires a careful materials design to minimize non-radiative VOC losses. Materials design rules play a key role in enabling high performance in organic photovoltaics. Here the authors achieve 12.25% efficiency on 1 cm2 non-fullerene solar cells by tuning the side chains’ branching point and the fluorine substitutions in donor and acceptor materials.

Abstract Solar cells with polysilicon-based passivating and carrier selective contacts are in the nascent stages of industrialization. Advances in commercially available metallization pastes are necessary to further improve the efficiency of industrial polysilicon-based solar cells and approach the best-reported lab efficiency of such cells. In this work, we have analyzed the front and rear screen-printed metallization of large-area n-type monoPolyTM cells with rear-side low-pressure chemical vapor deposited (LPCVD) polysilicon. Metallization pastes specifically optimized for contacting poly-Si layers are used in this work. The metallization is evaluated by studying its impact on the solar cell performance and by further electrical and optical characterization. An efficiency improvement of 0.5% absolute is demonstrated due to improvements in metallization pastes with a 23% champion cell efficiency (cell area: 244.3 cm2, busbarless metallization). An analysis of the fabricated cells indicates that the efficiency is currently limited by recombination at the front surface, both at the passivated and the metallized regions.

A non-fullerene acceptor with a high relative dielectric constant (εr) over 9 is developed. It offers an efficiency of 8.5%, which is the best result for organic solar cells employing high εr materials. Further research should focus on morphology optimization to make high εr practically useful in devices.

By processing PTzBI:N2200 blends with green solvent 2-methyltetrahydrofuran, all-PSCs with a power conversion efficiency over 9% can be achieved.

AbstractThe design and synthesis of three n‐type conjugated polymers based on a naphthalene diimide–thiophene skeleton are presented. The control polymer, PNDI‐2HD, has two identical 2‐hexyldecyl side chains, and the other polymers have different alkyl side chains; PNDI‐EHDT has a 2‐ethylhexyl and a 2‐decyltetradecyl side chain, and PNDI‐BOOD has a 2‐butyloctyl and a 2‐octyldodecyl side chain. These copolymers with different alkyl side chains exhibit higher melting and crystallization temperatures, and stronger aggregation in solution, than the control copolymer PNDI‐2HD that has the same side chain. Polymer solar cells based on the electron‐donating copolymer PTB7‐Th and these novel copolymers exhibit nearly the same open‐circuit voltage of 0.77 V. Devices based on the copolymer PNDI‐BOOD with different side chains have a power‐conversion efficiency of up to 6.89%, which is much higher than the 4.30% obtained with the symmetric PNDI‐2HD. This improvement can be attributed to the improved charge‐carrier mobility and the formation of favorable film morphology. These observations suggest that the molecular design strategy of incorporating different side chains can provide a new and promising approach to developing n‐type conjugated polymers.