Lijian Zuo

Enhanced Photovoltaic Performance of CH3NH3PbI3 Perovskite Solar Cells through Interfacial Engineering Using Self-Assembling Monolayer

Morphology control is critical to achieve high efficiency CH3NH3PbI3 perovskite solar cells (PSC). The surface properties of the substrates on which crystalline perovskite thin films form are expected to affect greatly the crystallization and, thus, the resulting morphology. However, this topic is seldom examined in PSC. Here we developed a facile but efficient method of modifying the ZnO-coated substrates with 3-aminopropanioc acid (C3-SAM) to direct the crystalline evolution and achieve the optimal morphology of CH3NH3PbI3 perovskite film. With incorporation of the C3-SAM, highly crystalline CH3NH3PbI3 films were formed with reduced pin-holes and trap states density. In addition, the work function of the cathode was better aligned with the conduction band minimum of perovskite for efficient charge extraction and electronic coupling. As a result, the PSC performance remarkably increased from 9.81(±0.99)% (best 11.96%) to 14.25(±0.61)% (best 15.67%). We stress the importance of morphology control through substrate surface modification to obtain the optimal morphology and device performance of PSC, which should generate an impact on developing highly efficient PSC and future commercialization.

Design of a versatile interconnecting layer for highly efficient series-connected polymer tandem solar cells

A versatile interconnecting layer (ICL) based on reflective ultra-thin Ag (8–14 nm) was developed to enable the fabrication of a series-connected micro-cavity tandem polymer solar cell. This novel ICL can manipulate the optical field distribution between the constituent sub-cells to address the challenge of current matching. As a result, a very high power conversion efficiency (∼11%) and high summed external quantum efficiency of >90% were demonstrated.

Microcavity‐Enhanced Light‐Trapping for Highly Efficient Organic Parallel Tandem Solar Cells

A high-performance parallel tandem solar cell employing ultra-thin Ag as the intermediate anode is demonstrated, which comprises a semitransparent front sub-cell and a microcavity assisted back sub-cell. In addition to the extended optical field as a result of the tandem architecture, the prominent microcavity resonance formed in the back sub-cell enables such a parallel tandem configuration to possess high light utilization efficiency (the peak EQE value is over 80%) and a high photovoltaic performance of 9.2%. This study establishes an effective architecture that can be generally applicable to all organic materials for improving their performance.

Star-Shaped D–A Small Molecules Based on Diketopyrrolopyrrole and Triphenylamine for Efficient Solution-Processed Organic Solar Cells

Three star-shaped D-A small molecules, (P-DPP)(3)TPA, (4-FP-DPP)(3)TPA, and (4-BuP-DPP)(3)TPA were designed and synthesized with triphenylamine (TPA) as the core, diketopyrrolopyrrole (DPP) as the arm, and unsubstituted or substituted benzene rings (phenyl, P; 4-fluoro-phenyl, 4-FP; 4-n-butyl-phenyl, 4-BuP) as the end-group. All the three small molecules show relatively narrow optical band gaps (1.68-1.72 eV) and low-lying highest occupied molecular orbital (HOMO) energy levels (-5.09∼-5.13 eV), implying that they are potentially good electron donors for organic solar cells (OSCs). Then, photovoltaic properties of the small molecules blended with [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(61)BM) as electron acceptor were investigated. Among three small molecules, the OSC based on (P-DPP)(3)TPA:PCBM blend exhibits a best power conversion efficiency (PCE) of 2.98% with an open-circuit voltage (V(oc)) of 0.72 V, a short-circuit current density (J(sc)) of 7.94 mA/cm(2), and a fill factor (FF) of 52.2%, which may be ascribed to the highest hole mobility of (P-DPP)(3)TPA.

Interfacial engineering of self-assembled monolayer modified semi-roll-to-roll planar heterojunction perovskite solar cells on flexible substrates

The morphologies of the perovskite (e.g. CH3NH3PbI3) layer are demonstrated to be critically important for highly efficient perovskite solar cells. This work applies 3-aminopropanoic acid as a self-assembled monolayer (C3-SAM) on a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transport layer (HTL) to modify the crystallinity and coverage of the CH3NH3PbI3−xClx film, resulting in a much smoother perovskite surface morphology together with a PCE increase from 9.7% to 11.6%. Since all fabrication steps of these inverted structure devices are carried out under low temperature conditions (processing temperature < 120 °C), it is possible to employ this method on flexible polymer substrates using roll-coating for the layer deposition. The roll-coated perovskite film on C3-SAM modified PEDOT:PSS presents a similar trend of improvement and results in enhanced PCE from 3.7% to 5.1%. The successful application of the facile HTL modification indicates a common strategy for SAM material design and selection for efficiency enhancement in perovskite photovoltaic devices.

Nonfullerene Tandem Organic Solar Cells with High Open‐Circuit Voltage of 1.97 V

Small-molecule nonfullerene-based tandem organic solar cells (OSCs) are fabricated for the first time by utilizing P3HT:SF(DPPB)4 and PTB7-Th:IEIC bulk heterojunctions as the front and back subcells, respectively. A power conversion efficiency of 8.48% is achieved with an ultrahigh open-circuit voltage of 1.97 V, which is the highest voltage value reported to date among efficient tandem OSCs.

Improved photon-to-electron response of ternary blend organic solar cells with a low band gap polymer sensitizer and interfacial modification

Incorporating two polymer donors with different bandgaps to compose a ternary blend bulk heterojunction (BHJ) is proved to be an effective approach to improve the device performance of BHJ polymer solar cells (PSCs). Here, we demonstrate an efficient ternary PSC consisting of a polythieno[3,4-b]-thiophene/benzodithiophene (PTB7):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) host blend sensitized by a low band gap (LBG) polymer poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:20,30-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3-benzothiadiazole)] (PDTP-DFBT). The addition of the PDTP-DFBT sensitizer remarkably extended the PSC photon to electron response from 750 to 900 nm, which increased the Jsc from 15.12 to 16.27 mA cm−2, and the device performance from 8.08% to 8.63%. A study on the morphology involving the atomic force microscopy mapping and grazing incident X-ray diffraction showed that the incorporation of PDTP-DFBT improved the crystallinity of the PTB7 film with most of the sensitizers associated with the PTB7 domains when blending with a PC71BM film. This observation, together with the unchanged Voc of the ternary PSC, implies a sensitizing mechanism with addition of PDTP-DFBT. After further interfacial modification with a capronic acid self-assembling monolayer (C3-SAM), a higher PCE of 9.06% was achieved, which is among the highest values of efficient ternary PSCs. Our work suggests that a sensitizing mechanism of ternary blends compensates for the light absorbing limitation of binary blend PSCs for high device performance.

Incorporation of ester groups into low band-gap diketopyrrolopyrrole containing polymers for solar cell applications

To increase the open circuit voltage (VOC) of polymer solar cells based on diketopyrrolopyrrole (DPP) containing polymers, the weakly electron-withdrawing thiophene-3,4-dicarboxylate unit was introduced into the polymer backbone. Two ester group functionalized DPP containing polymers, PCTDPP with a random structure and PDCTDPP with a regular structure, were designed and synthesized by the Stille coupling reaction. The resulting copolymers exhibit broad and strong absorption bands from 350 to 1000 nm with low optical band gaps below 1.40 eV. Through cyclic voltammetry measurements, it is found that regular PDCTDPPs HOMO energy level is 0.18 V lower than that of the corresponding random PCTDPP (−5.14 eV for PCTDPP and −5.32 eV for PDCTDPP). Preliminary photovoltaic properties of the copolymers blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor were investigated. The PSC based on a PCTDPP:PCBM blend shows a power conversion efficiency (PCE) up to 3.52%, with a VOC of 0.66 V, a short circuit current (ISC) of 8.53 mA cm−2, and a fill factor (FF) of 0.63. For the PDCTDPP:PCBM blend, the highest VOC reaches a value of 0.84 V, and a final PCE (0.92%) is limited by the poor hole mobility of the active layer.

Dithienopicenocarbazole Based Acceptors for Efficient Organic Solar Cells with Optoelectronic Response Over 1000 nm and an Extremely Low Energy Loss.

Two cheliform non-fullerene acceptors, DTPC-IC and DTPC-DFIC, based on a highly electron-rich core, dithienopicenocarbazole (DTPC), are synthesized, showing ultra-narrow bandgaps (as low as 1.21 eV). The two-dimensional nitrogen-containing conjugated DTPC possesses strong electron-donating capability, which induces intense intramolecular charge transfer and intermolecular π-π stacking in derived acceptors. The solar cell based on DTPC-DFIC and a spectrally complementary polymer donor, PTB7-Th, showed a high power conversion efficiency of 10.21% and an extremely low energy loss of 0.45 eV, which is the lowest among reported efficient OSCs.

Thiocyanate assisted performance enhancement of formamidinium based planar perovskite solar cells through a single one-step solution process

Thiocyanate ammonium (NH4SCN) is introduced into the fabrication of formamidinium lead triiodide (FAPbI3) films through one-step spin-coating. The promoted formation of black trigonal phase α-FAPbI3 with better crystallinity has been observed after the addition of NH4SCN, together with the supression of the formation of yellow hexagonal phase δ-FAPbI3. Planar perovskite solar cells (PVSCs) based on NH4SCN-assisted formed α-FAPbI3 films with high quality present a highest power conversion efficiency of 11.44% when 30 mol% NH4SCN is applied. Notably, the addition of NH4SCN is found to enhance the moisture stability of the perovskite. As a result, the planar PVSCs with 30 mol% NH4SCN additive show improved stability under ambient conditions (RH: 30–40%) over those based on pristine FAPbI3. NH4SCN simultaneously enhances the efficiency and moisture stability of FAPbI3 based PVSCs through a single one-step solution method, facilitating their commercial fabrication and application.