Swaminathan Venkatesan

Strategic review of secondary phases, defects and defect-complexes in kesterite CZTS–Se solar cells

Earth abundant kesterite copper-zinc-tin-sulfide–selenide (CZTS–Se) is considered as cost-effective material for next generation solar cells. However, current CZTS–Se solar cells have much lower efficiency than CIGS solar cells. Rapid progress in achieving the target efficiency in CZTS–Se solar cells is hindered by the narrow phase stability of the quaternary phase, Cu2ZnSn(SxSe1−x)4, and the existence of other competitive and complex secondary phases and defects. This resulted in structural inhomogeneity, local fluctuation of open circuit voltage and high carrier recombination that finally lead to poor device performance and repeatability issues. The higher performance of off-stoichiometric CZTS materials, copper-poor and zinc-rich, and their inherent association with secondary phases and defects force the scientific community to investigate them together. This work aims to provide a comprehensive review for optimum growth conditions to achieve efficient kesterite CZTS–Se material under different conditions, complementary characterization techniques to detect unwanted phases, defects and defect-complexes and various approaches to reduce the secondary phases, defects and defect-complexes for higher performance in CZTS–Se solar cells. Understanding and addressing the structural inhomogeneity, control growth and material characterization are expected to yield closer performance parity between CZTS–Se and CIGS solar cells.

Interfacial Study To Suppress Charge Carrier Recombination for High Efficiency Perovskite Solar Cells.

We report effects of an interface between TiO2-perovskite and grain-grain boundaries of perovskite films prepared by single step and sequential deposited technique using different annealing times at optimum temperature. Nanoscale kelvin probe force microscopy (KPFM) measurement shows that charge transport in a perovskite solar cell critically depends upon the annealing conditions. The KPFM results of single step and sequential deposited films show that the increase in potential barrier suppresses the back-recombination between electrons in TiO2 and holes in perovskite. Spatial mapping of the surface potential within perovskite film exhibits higher positive potential at grain boundaries compared to the surface of the grains. The average grain boundary potential of 300-400 mV is obtained upon annealing for sequentially deposited films. X-ray diffraction (XRD) spectra indicate the formation of a PbI2 phase upon annealing which suppresses the recombination. Transient analysis exhibits that the optimum device has higher carrier lifetime and short carrier transport time among all devices. An optimum grain boundary potential and proper band alignment between the TiO2 electron transport layer (ETL) and the perovskite absorber layer help to increase the overall device performance.

Enhanced charge transport and photovoltaic performance of PBDTTT-C-T/PC70BM solar cells via UV–ozone treatment

In this work, the electron transport layer of PBDTTT-C-T/PC70BM polymer solar cells were subjected to UV-ozone treatment, leading to improved cell performances from 6.46% to 8.34%. The solar cell efficiency reached a maximum of 8.34% after an optimal 5 minute UV-ozone treatment, and then decreased if treated for a longer time. To the best of our knowledge, the mechanism behind the effects of UV-ozone treatment on the improvement of charge transport and cell performance is not fully understood. We have developed a fundamental understanding of the UV-ozone treatment mechanism, which explains both the enhancements in charge transport and photovoltaic performance at an optimal treatment time, and also the phenomenon whereby further treatment time leads to a drop in cell efficiency. Transient photocurrent measurements indicated that the cell charge transport times were 1370 ns, 770 ns, 832 ns, 867 ns, and 1150 ns for the 0 min, 5 min, 10 min, 15 min, and 20 min UV-ozone treatment times, respectively. Therefore the 5 min UV-ozone treatment time led to the shortest transport time and the most efficient charge transport in the cells. The 5 min UV-ozone treated sample exhibited the highest peak intensity (E2) in the Raman spectra of the treated films, at about 437 cm(-1), indicating that it possessed the best wurtzite phase crystallinity of the ZnO films. Further increasing the UV-ozone treatment time from 5 to 20 min induced the formation of p-type defects (e.g. interstitial oxygen atoms), pushing the ZnO Fermi-level further away from the vacuum level, and decreasing the wurtzite crystallinity.

Oleamide as a self-assembled cathode buffer layer for polymer solar cells: the role of the terminal group on the function of the surfactant

An amphiphilic surfactant oleamide was incorporated into P3HT:PCBM bulk heterojunction polymer solar cells (BHJ-PSCs) as a novel cathode buffer layer (CBL) for the first time by doping in the P3HT:PCBM photoactive layer followed by self-assembly. The power conversion efficiency (PCE) of the annealed P3HT:PCBM/oleamide BHJ-PSC device is enhanced by ∼28% at the optimum oleamide doping ratio of 2.5%, which is primarily due to the increase of fill factor (FF) by ∼22%. The surface morphologies of the oleamide-incorporated P3HT:PCBM photoactive films were studied by transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning Kelvin probe microscopy (SKPM), revealing that oleamide molecules initially doped in the P3HT:PCBM layer may undergo self-assembly and migrate to the top surface of the P3HT:PCBM layer, leading to the formation of a cathode buffer layer (CBL) as an interfacial dipole layer between the photoactive layer and Al cathode electrode. Such an oleamide interfacial dipole layer lowers the work function of Al, thus the energy level offset between the work function of Al and the LUMO level of the PCBM acceptor is decreased, facilitating the electron extraction by the Al cathode. Furthermore, we found that the crystallinity of P3HT upon the incorporation of oleamide was almost unchanged according to X-ray diffraction (XRD) characterization. It is noteworthy that, this phenomenon is completely different from the case of the previously reported analogous surfactant oleic acid, which was doped in the P3HT:PCBM photoactive layer and led to the efficiency enhancement as well due to the increased crystallinity of P3HT, suggesting the strong influence of the terminal group of the surfactant on its function in P3HT:PCBM BHJ-PSC devices.

Benzothiadiazole-based polymer for single and double junction solar cells with high open circuit voltage.

Single and double junction solar cells with high open circuit voltage were fabricated using poly{thiophene-2,5-diyl-alt-[5,6-bis(dodecyloxy)benzo[c][1,2,5]thiadiazole]-4,7-diyl} (PBT-T1) blended with fullerene derivatives in different weight ratios. The role of fullerene loading on structural and morphological changes was investigated using atomic force microscopy (AFM) and X-ray diffraction (XRD). The XRD and AFM measurements showed that a higher fullerene mixing ratio led to breaking of inter-chain packing and hence resulted in smaller disordered polymer domains. When the PBT-T1:PC60BM weight ratio was 1 : 1, the polymer retained its structural order; however, large aggregated domains formed, leading to poor device performance due to low fill factor and short circuit current density. When the ratio was increased to 1 : 2 and then 1 : 3, smaller amorphous domains were observed, which improved photovoltaic performance. The 1 : 2 blending ratio was optimal due to adequate charge transport pathways giving rise to moderate short circuit current density and fill factor. Adding 1,8-diiodooctane (DIO) additive into the 1 : 2 blend films further improved both the short circuit current density and fill factor, leading to an increased efficiency to 4.5% with PC60BM and 5.65% with PC70BM. These single junction solar cells exhibited a high open circuit voltage at ∼ 0.9 V. Photo-charge extraction by linearly increasing voltage (Photo-CELIV) measurements showed the highest charge carrier mobility in the 1 : 2 film among the three ratios, which was further enhanced by introducing the DIO. The Photo-CELIV measurements with varying delay times showed significantly higher extracted charge carrier density for cells processed with DIO. Tandem devices using P3HT:IC60BA as bottom cell and PBT-T1:PC60BM as top cell exhibited a high open circuit voltage of 1.62 V with 5.2% power conversion efficiency.

Enhanced Lifetime of Polymer Solar Cells by Surface Passivation of Metal Oxide Buffer Layers

The role of electron selective interfaces on the performance and lifetime of polymer solar cells were compared and analyzed. Bilayer interfaces consisting of metal oxide films with cationic polymer modification namely poly ethylenimine ethoxylated (PEIE) were found to enhance device lifetime compared to bare metal oxide films when used as an electron selective cathode interface. Devices utilizing surface-modified metal oxide layers showed enhanced lifetimes, retaining up to 85% of their original efficiency when stored in ambient atmosphere for 180 days without any encapsulation. The work function and surface potential of zinc oxide (ZnO) and ZnO/PEIE interlayers were evaluated using Kelvin probe and Kelvin probe force microscopy (KPFM) respectively. Kelvin probe measurements showed a smaller reduction in work function of ZnO/PEIE films compared to bare ZnO films when aged in atmospheric conditions. KPFM measurements showed that the surface potential of the ZnO surface drastically reduces when stored in ambient air for 7 days because of surface oxidation. Surface oxidation of the interface led to a substantial decrease in the performance in aged devices. The enhancement in the lifetime of devices with a bilayer interface was correlated to the suppressed surface oxidation of the metal oxide layers. The PEIE passivated surface retained a lower Fermi level when aged, which led to lower trap-assisted recombination at the polymer-cathode interface. Further photocharge extraction by linearly increasing voltage (Photo-CELIV) measurements were performed on fresh and aged samples to evaluate the field required to extract maximum charges. Fresh devices with a bare ZnO cathode interlayer required a lower field than devices with ZnO/PEIE cathode interface. However, aged devices with ZnO required a much higher field to extract charges while aged devices with ZnO/PEIE showed a minor increase compared to the fresh devices. Results indicate that surface modification can act as a suitable passivation layer to suppress oxidation in metal oxide thin films for enhanced lifetime in inverted organic solar cells.

Room temperature, air crystallized perovskite film for high performance solar cells

For the first time, room temperature heating free growth and crystallization of perovskite films in ambient air without the use of thermal annealing is reported. Highly efficient perovskite nanorod-based solar cells were made using ITO/PEDOT:PSS/CH3NH3PbI3 nanorods/PC60BM/rhodamine/Ag. All the layers except PEDOT:PSS were processed at room temperature thereby eliminating the need for thermal treatment. Perovskite films were spin coated inside a N2 filled glovebox and immediately were taken outside in air having 40% relative humidity (RH). Exposure to humid air was observed to promote the crystallization process in perovskite films even at room temperature. Perovskite films kept for 5 hours in ambient air showed nanorod-like morphology having high crystallinity, with devices exhibiting the highest PCE of 16.83%, which is much higher than the PCE of 11.94% for traditional thermally annealed perovskite film based devices. It was concluded that moisture plays an important role in room temperature crystallization of pure perovskite nanorods, showing improved optical and charge transport properties, which resulted in high performance solar cells.

Kelvin probe force microscopic imaging of the energy barrier and energetically favorable offset of interfaces in double-junction organic solar cells.

A double-junction polymer solar cell (PSC) has attracted extensive attention as a promising approach to increasing efficiency. Tunneling/recombination interlayers between subcells play a critical role in double-junction PSCs. Interlayers include electron-transport layers (ETLs) such as Nb₂O₅, ZnO, and TiO(x) and hole-transport layers (HTLs) including PEDOT:PSS. These materials have all been used as interlayer materials, but it remains unclear which one is better than the other. Kelvin probe force microscopy (KFM) was used to identify the energy barrier and energetically favorable energy offset at the interfaces of acceptor-ETL (e.g., PCBM-Nb₂O₅, PCBM-ZnO, and PCBM-TiO(x)) and donor-HTL (e.g., MDMO-PPV/PEDOT:PSS). Here the interface refers to the junction of two materials, formed by drop-casting one on top of other. The interface is buried and is therefore not accessible to the KFM probe. The energy barrier for electron transport from PCBM to ETL was found at ∼0.20, ∼0.12, and ∼0.012 eV at the PCBM-Nb₂O₅, PCBM-ZnO, and PCBM-TiO(x) interfaces, respectively. Hole transport from the donor polymer to PEDOT:PSS was found to be energetically favorable with an energy offset of ∼0.14 eV to facilitate hole transport. The thickness independences of the energy barrier and energetically favorable energy offset at the interfaces of acceptor-ETL and donor-HTL were also observed. This work will provide guidance for researchers to identify and select appropriate materials as interlayers in double-junction PSCs.

Improved performance by morphology control via fullerenes in PBDT-TBT-alkoBT based organic solar cells

In this work, we report improved performance by controlling morphology using different fullerene derivatives in poly{2-octyldodecyloxy-benzo[1,2-b;3,4-b]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(thieno[3,2-b]thiophen-2-yl)-benzo[c][1,2,5]thiadiazole} (PBDT-TBT-alkoBT) based organic solar cells. PC60BM and PC70BM fullerenes were used to investigate the characteristic changes in morphology and device performance. Fullerenes affect device efficiency by changing the active layer morphology. PC70BM with broader absorption than PC60BM resulted in reduced device performance which was elucidated by the intermixed granular morphology separating each larger grain in the PC70BM/polymer composite layer which created a higher density of traps. However after adding additive 1,8-diiodooctane (DIO), a fibrous morphology was observed due to the reduced solubility of the polymer and increased solubility of PC70BM in chloroform. The fibrous morphology improved charge transport leading to an increase in overall device performance. Atomic force microscopy (AFM), photo-induced charge extraction by linearly increasing voltage (photo-CELIV), and Kelvin probe force microscopy (KPFM) were used to investigate the nanoscale morphology of the active layer with different fullerene derivatives. For the PC60BM based active layer, AFM images revealed a dense fibrous morphology and more distinct fibrous morphology was observed by adding DIO. The PC70BM based active layer only exhibited an intermixed granular morphology instead of a fibrous morphology observed in the PC60BM based active layer. However, addition of DIO into the PC70BM based active layer led to fibrous morphology. When additive DIO was not used, a wider distribution of surface potential was observed for PC70BM than the PC60BM based active layer by KPFM measurements, indicating that polymer and fullerene domains are separated. When DIO was used, a narrower distribution of surface potential for both PC70BM and PC60BM based active layers was observed. Photo-CELIV experiments showed larger extracted charge carrier density and mobility in the PC70BM/DIO film.

Morphological Evolution and Its Impacts on Performance of Polymer Solar Cells

In this paper, the role of fullerene loading on the nanomorphology and photovoltaic performance of alternating copolymer poly{2-octyldodecyloxy-benzo[1,2-b;3,4-b] dithiophene-alt-5,6-bis(dodecyloxy)-4,7bis(thiophen-2-yl)-benzo[c] [1,2,5]-thiadiazole} (PBDT-ABT-1) blend films was investigated. The morphology of blend films with different Phenyl C-60-butyric acid methyl ester (PCBM) mixing ratios and solvent additives was studied using atomic force microscopy (AFM) and energy-filtered transmission electron microscopy (EFTEM). AFM and EFTEM images showed difference in the intermixing of polymer with fullerene between 1:1, 1:2, and 1:3 weight ratios. Polymer/PCBM intermixed domain size increases with higher PCBM weight ratios. X-ray diffraction measurements on the pristine polymer and blend films cast without additives did not show any peaks, suggesting an amorphous nature of PBDT-ABT-1. EFTEM images from the donor/acceptor composite showed intermixed polymer-PCBM domains separated by the polymer boundary. Furthermore, EFTEM images for di-iodooctane (DIO) additive cast film revealed purer polymer domain. Photo-charge extraction by linearly increasing voltage measurement exhibited that charge extraction is highest in the nanomorphology sample with a weight ratio of 1:2, corresponding to the lowest bimolecular recombination and the highest charge carrier mobility.