- Degradation of PTB7:PC70BM solar cells characterized by Impedance Spectroscopy- STSM Ref No:24796

Transcription

1 Stable Next-Generation Photovoltaics: Unravelling Degradation Mechanisms of Organic Solar Cells by Complementary Characterization Techniques StableNextSol MP Degradation of PTB7:PC70BM solar cells characterized by Impedance Spectroscopy- STSM Ref No:24796 G. del Pozo 1, B. Arredondo 1, G. Susanna 2, F. Brunetti 2, and B. Romero 1 1 Dpto. Tecnología Electrónica, Escuela Superior de Ciencias Experimentales y Tecnología, Universidad Rey Juan Carlos, C\ Tulipán s\n, 28933, Móstoles, Madrid, Spain 2 CHOSE (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering, University of Rome Tor Vergata, via del Politecnico 1, Rome, Italy COST is supported by the EU Framework Programme Horizon th MC Meeting, 3 rd WG Meeting Vilnius, Lithuania. October 19 th - 20 th 2015

2 TITLE: Degradation of PTB7:PC70BM solar cells characterized by Impedance Spectroscopy HOST Laboratory : CHOSE, University of Rome Tor Vergata, Rome, Italy GUEST Laboratory: Universidad Rey Juan Carlos, Madrid, Spain ESR Name: Gonzalo Del Pozo Duration: 2 months Objectives: Study the degradation of encapsulated PTB7:PC70BM solar cells with PEIE as HBL using Impedance Spectroscopy 2

3 Impedance Spectroscopy (IS) V DC Low Frequency (~Hz) High Frequency (~MHz) I t = ID C + i (t) V V t = VD C + v(t) Z t = v t i t = Z + iz Z t Z t Resistence Reactance 3

4 Fabrication and characterization ITO/ PEIE/ PTB7:PC 70 BM/ MoO 3 /Ag Active layer thickness ~100 nm. Cathode thickness of ~5/100 nm Fabrication process and encapsulation in Glove-box Efficiency ~9% Solar Simulator 4

5 IS Measurements Samples stored in dark conditions outside the glove-box >2 months. Measures once a week. The devices are illuminated several minutes in order to activate the PEIE layer, until the S-shape disappears and their efficiency stabilizes. IS measurements are carried out under dark conditions. Freq: 1 Hz Hz; V: 0 V-0.8 V Model used to simulate the data, proposed by Guerrero et al 1 with the addition of an inductance component. 1 Guerrero, A., Ripolles-Sanchis, T., Boix, P. P. and Garcia-Belmonte, G. Series resistance in organic bulk-heterojunction solar devices: Modulating carrier transport with fullerene electron traps. Organic Electronics, 13(11), (2012). 5

6 Chosen circuit: RL-R-RCPE L R S R T R P τ REC =R P CPE T LF- Recombination HF- Diffusion C T CPE τ DIF =R T CPE T µ= L2 τ DIF V T 6

7 Results Impedance under dark conditions of the first (left) and last day of measurements (right) 7

8 Decrease in R P at low voltages and the increase at high voltages electrical degradation of the cell. Rise of the ratio R T /R P shows the enhancement of the diffusion mechanism, which causes the worsening of charge transport. 8

9 τ DIF =R T CPE T τ REC =R P CPE T µ= L2 τ DIF V T Geometrical capacitance (C T ) remains constant along all the days, as was expected. The increase of CPE T reflects a poor charge extraction through the electrodes. τ DIFF increase lower mobility τ REC decreases for low voltages and increases for high voltages, reflecting the change in R P. 9

10 Resonance Frequency Observable at high Frequencies f>f Res 1 Resonance Frequency: 2π LC 105 Hz L=3.5 µh 10

11 Conclusions Degradation of PTB7:PC70BM devices has been studied using IS technique. Data has been fitted using a standard circuit with a series inductance. The evolution of the R P shows an electric degradation of the device: at low V the resistance decrease due to the increase of the leakage currents while at high V the value increase, due to the series resistance. The rise of the ratio R T /R P shows an enhancement of the transport mechanism, that can be also seen in the decrease of the mobility. In the case of CPE T a large increase reflects a worsening of the charge extraction. Finally the values of C T and L remain almost unchanged for all the voltages, being all the values in the same order of magnitude for all the voltages and days. 11