Study of thin film poly-crystalline CdTe solar cells presenting high acceptor concentrations achieved by in-situ arsenic doping

Doping of CdTe using Group-V elements (As, P, and Sb) has gained interest in pursuit of increasing the cell voltage of CdTe thin film solar devices. Studies on bulk CdTe crystals have shown that much higher acceptor concentration than the traditional copper treatment is possible with As, P or Sb, enabled by high process temperature and/or rapid thermal quenching under Cd overpressure. We report a comprehensive study on in-situ As doping of poly-crystalline CdTe solar cells by MOCVD, whereby high acceptor densities, approaching 3 × 1016 cm−3 were achieved at low growth temperature of 390 °C. No As segregation could be detected at grain boundaries, even for 1019 As cm−3. A shallow acceptor level (+0.1 eV) due to AsTe substitutional doping and deep-level defects were observed at elevated As concentrations. Devices with variable As doping were analysed. Narrowing of the depletion layer, enhancement of bulk recombination, and reduction in device current and red response, albeit a small near infrared gain due to optical gap reduction, were observed at high concentrations. Device modelling indicated that the properties of the n-type window layer and associated interfacial recombination velocity are highly critical when the absorber doping is relatively high, demonstrating a route for obtaining high cell voltage

Effects of PCBM loading and thermal annealing on nanomorphology of blend of polymer/fullerene thin films solar cells: Impact on charge carrier mobility and efficiency

Blend of P3HT/Fullerene thin films solar cell with two different percentage ratio of PCBM loading is investigated. Optical absorption spectroscopy is employed to elucidate the nature of PCBM cluster formation upon thermal annealing. Sandwich structures comprising of ITO/ Cs2CO3/ P3HT: PCBM/ LiF/ Al (electron only device), and ITO/ PEDOT:PSS/ P3HT:PCBM/ Au (hole only device) are fabricated using spin coating for the investigations concerning electron and hole mobilities. The impact of charge carrier mobilities on bimolecular recombination and ultimately the power conversion efficiency for two different PCBM loading is also investigated. A direct correlation between Langev in recombination rate and short circuit current density as a function of thermal annealing is realized. The maximum power conversion efficiency is measured at 150°C for P3HT: PCBM (1:1) solar cell. © (2014) Trans Tech Publications, Switzerland

Impedance spectroscopy and capacitance – voltage measurements analysis: Impact of charge carrier lifetimes and mapping vertical segregation in bulk heterojunction P3HT: PCBM solar cells

Impedance spectroscopy measurement is employed to study the impact of thermal annealing on charge carrier lifetimes in P3HT: PCBM bulk heterojunction solar cells. Upon thermal annealing at 150 °C, a correlation between charge carrier lifetime and device performance is identified. The best power conversion efficiency reported here corresponds to the devices annealed at 150 °C, yielding the longest charge carrier life time. In addition to lateral segregation, thermal annealing promotes accumulation of PCBM molecular aggregates towards the cathode inducing vertical segregation as is evident from the analysis of results obtained from capacitance – voltage measurements. Based on Mott-Schottky relation, the measured values of the flat band potential as a function of thermal annealing is correlated to the concentration of PCBM at cathode interface and a compositional gradient profile induced by thermal annealing is proposed

Study of thin film poly-crystalline CdTe solar cells presenting high acceptor concentrations achieved by in-situ arsenic doping

© 2019 Doping of CdTe using Group-V elements (As, P, and Sb) has gained interest in pursuit of increasing the cell voltage of CdTe thin film solar devices. Studies on bulk CdTe crystals have shown that much higher acceptor concentration than the traditional copper treatment is possible with As, P or Sb, enabled by high process temperature and/or rapid thermal quenching under Cd overpressure. We report a comprehensive study on in-situ As doping of poly-crystalline CdTe solar cells by MOCVD, whereby high acceptor densities, approaching 3 × 10 16 cm −3 were achieved at low growth temperature of 390 °C. No As segregation could be detected at grain boundaries, even for 10 19 As cm −3 . A shallow acceptor level (+0.1 eV) due to As Te substitutional doping and deep-level defects were observed at elevated As concentrations. Devices with variable As doping were analysed. Narrowing of the depletion layer, enhancement of bulk recombination, and reduction in device current and red response, albeit a small near infrared gain due to optical gap reduction, were observed at high concentrations. Device modelling indicated that the properties of the n-type window layer and associated interfacial recombination velocity are highly critical when the absorber doping is relatively high, demonstrating a route for obtaining high cell voltage

Development of arsenic doped Cd(Se,Te) absorbers by MOCVD for thin film solar cells

Recent developments in CdTe solar cell technology have included the incorporation of ternary alloy Cd(Se,Te) in the devices. CdTe absorber band gap grading due to Se alloying contributes to current density enhancement and can result in device performance improvement. Here we report Cd(Se,Te) polycrystalline thin films grown by a chamberless inline atmospheric pressure metal organic chemical vapour deposition technique, with subsequent incorporation in CdTe solar cells. The compositional dependence of the crystal structure and optical properties of Cd(Se,Te) are examined. Selenium graded Cd(Se,Te)/CdTe absorber structure in devices are demonstrated using either a single CdSe layer or CdSe/Cd(Se,Te) bilayer (with or without As doping in the Cd(Se,Te) layer). Cross-sectional TEM/EDS, photoluminescence spectra and secondary ion mass spectroscopy analysis confirmed the formation of a graded Se profile toward the back contact with a diffusion length of ∼1.5 μm and revealed back-diffusion of Group V (As) dopants from the CdTe layer into Cd(Se,Te) grains. Due to the strong Se/Te interdiffusion, CdSe in the Se bilayer configuration was unable to form an n-type emitter layer in processed devices. In situ As doping of the Cd(Se,Te) layer benefited the device junction quality with current density reaching 28.3 mA/cm 2 . The results provide useful insights for the optimisation of Cd(Se,Te)/CdTe solar cells

A quantitative study of the formation of PCBM clusters upon thermal annealing of P3HT/PCBM bulk heterojunction solar cell

Several thermal annealing strategies are employed to account for the formation of PCBM aggregates in a blend of P3HT/PCBM thin films. Optical absorption spectroscopy reveals dependence of PCBM cluster formation upon different annealing strategies as indicated by optical microscopic images. Using Raman image mapping of the selected locations we were able to identify PCBM rich regions. Exciton generation rate as a function of thermal annealing is in good correlation with short circuit current density. The probability of exciton dissociation as a function of thermal annealing reveals an almost static behaviour in response to the temperature rise up to 75 °C. The enhanced intensity of photoluminescence signal upon thermal annealing supports the increase in exciton generation rates resulting from formation of PCBM clusters and recrystallisation of P3HT domains, removing nonradiative recombination centres. Current density voltage characterisation reveals dependence of device parameters on thermal annealing strategies

Efficient P3HT:PCBM bulk heterojunction organic solar cells; effect of post deposition thermal treatment

Organic solar cells based on P3HT:PCBM bulk heterojunction were prepared and subjected to post annealing at different temperatures (100, 120, 140, 160 and 180 °C). SEM, AFM as well as optical images have revealed that post deposition heat treatment has induced significant phase segregation between P3HT and PCBM which were found to result in growth of PCBM clusters on the films surface. The P3HT:PCBM absorption spectra were found to be blue shifted by 7 nm in films subjected to heat treatment at 160 °C and 180 °C. XRD data show a single diffraction peak at 2θ = 5.33 ± 0.23o for P3HT:PCBM films and was attributed to the edge-on arrangement of the (100) plane. Space charge limited conduction theory was employed to determine the charge carrier mobility; the highest obtained mobility was obtained for devices with active layers heat-treated at 140 °C. The change in the barrier height was derived from dark I–V. The variation in the metal–semiconductor contact between the Al electrode and P3HT:PCBM active layer were addressed and the barrier height has increased to form hole blocking contact and the ideality factor has decreased implying a decrease in the recombination rate. A direct relation between Fermi level, Vbi, and Voc was studied. Efficient device performance was ascribed to P3HT:PCBM layers which were subjected to post deposition heat treatment at 140 °C with PCE = 5.5 %, FF = 65.6 %, Jsc = 12.9 mA cm−2 and Voc = 0.65 V

Impact of charge carrier mobility and electrode selectivity on the performance of organic solar cells

Low charge carrier mobilities as often observed for photoactive materials of organic solar cells have significant impact on their performance. They cause accumulation of charge carriers which can be described quantitatively by a nonohmic transport resistance in the framework of an analytical model. Further addressed in this work is surface recombination stemming from insufficient electrode selectivity which is another factor limiting the performance of organic solar cells