Chapter 2
Chapter 2
Figure 2.3. AFM topographic images of rrP3HT:PC71BM with (a-d) different Tan with constant 1:0.8 donor- acceptor molar mass ratio and (e-h) different donor-acceptor molar mass ratio and at constant Tan= 150°C.
2.2.2 Photovoltaic Characterizations
Figure. 2.4, 2.5 and 2.6 represents the photovoltaic properties of all the fabricated BHJ devices. In this study, total seventeen different device structures were systematically analysed which are listed in Table 2.1. From Figure. 2.4a it was observed that at room temperature with the variation of molar mass ratio of the blend active polymer very minor variation in current density can be achieved. From the device having configuration (1b), the highest PCE was observed to be ɳ=2.88% with Jsc=10.33 mA/cm2, Voc=0.53 V and FF=
52.17% with 1:0.8 as the molar mass ratio. In the next step, by keeping constant 1:0.8 molar mass ratio and varying the annealing temperature, it has been observed that only by changing the Tan from RT to 150°C, the PCE increases from ɳ=2.88% to 3.94% with Jsc=12.39 mA/cm2, Voc=0.54 V and FF= 58.41% [Figure. 2.4b and device configuration (2c)]. Figure. 2.4c signifies the variation of current density with respect to different molar mass ratio at constant Tan= 150°C. From this data it can be concluded that, the Tan= 150°C, and molar mass ratio= 1:0.8 are the optimum value for rrP3HT:PC71BM blend polymer where the obtained highest photovoltaic parameters are, PCE, ɳ=3.84% with Jsc=12.97 mA/cm2, Voc=0.55 V and FF= 53.81% [device configuration (3b)] without any contact modification. In the fourth step (Figure. 2.4d), with these optimum conditions, we studied the effect of different cathode buffer layers on the output performance of the cells.24-29 The energy band diagram of all these buffer layers along with the rrP3HT:PC71BM blend polymers are schematically described in Figure. 2.5.
Chapter 2
Figure 2.4. Current density vs. voltage (J-V) characteristics of rrP3HT:PC71BM with (a) different donor- acceptor molar mass ratio at RT, (b) at different Tan with constant 1:0.8 donor-acceptor molar mass ratio (c) different donor-acceptor molar mass ratio and at constant Tan= 150°C and (d) different cathode buffer layer and at optimum conditions.
Figure 2.5. Band energy diagrams of the fabricated devices having (a) LiF/Al, (b) Ca/Al, (c) Alq3/Al (d) BPhen/Al and (e) BCP/Al as cathode buffer layer.
Using Ca/Al as cathode contact, similar types of device output were obtained [ɳ
Ca/Al=3.20% having device configuration, (4b)] like LiF/Al [ɳ LiF/Al= 4.01% having device configuration, (4a)] since the work function of Ca/Al (2.9 eV) is almost similar like LiF/Al (3 eV) [Figure. 2.5a and Figure. 2.5b]. But with Ca/Al, the stability of the fabricated device was observed very poor compared to LiF/Al as Ca can be easily oxidized
Chapter 2
Figure 2.6. EQE spectra of rrP3HT:PC71BM with (a) different donor-acceptor molar mass ratio at RT, (b) at different Tan with constant 1:0.8 donor-acceptor molar mass ratio (c) different donor-acceptor molar mass ratio and at constant Tan= 150°C and (d) different cathode buffer layer and at optimum conditions.
Figure 2.7. Nyquist plots of rrP3HT:PC71BM with (a) different donor-acceptor molar mass ratio at RT, (b) at different Tan with constant 1:0.8 donor-acceptor molar mass ratio (c) different donor-acceptor molar mass ratio and at constant Tan= 150°C and (d) different cathode buffer layer and at optimum conditions.
in ambient atmosphere. In case of Alq3/Al [having device configuration (4c)], as Alq3 is an electron injecting layer, due to the interaction of the HOMO level of this buffer material with the blend active layer, some charge carriers recombination occur at the interface (Figure. 2.5c). As a result lower PCE value (ɳAlq3/Al=2.91%)
Chapter 2
was observed. For the devices having configuration (4d) and (4e), we used BPhen and BCP as cathode buffer layer along with Al as cathode contact respectively. In both the cases, better photovoltaic properties were observed because of superior hole blocking capacity of these molecules due to which less charge carriers are recombined at the blend polymer-cathode contact interface. Among these two configurations, BCP/Al showed better PCE of ɳBCP/Al = 4.79% with Jsc=14.21 mA/cm2, Voc=0.58 V and FF=57.8% compared to BPhen/Al (ɳBPhen/Al=4.36%) as BCP has higher band gap, due to which it shows higher selectivity towards electron and blocking the holes to minimize the charge recombination between the blend polymer-cathode contact compared to BPhen. As a result we get better band energy alignment which enhances the overall photovoltaic performance (Figure. 2.5d and Figure. 2.5e). The photovoltaic performance parameters of all the devices are summarized in Table 2.2.
Table 2.2. Summary of BHJ solar cell performance with rrP3HT:PC71BM as the active blend layer and with different variation.
Device
Configuration Jsc
(mA.cm-2) Voc
(V) FF (%) PCE,
(%)
(1)(a) 10.20 0.52 50.62 2.69
(1)(b) 10.33 0.53 52.17 2.88
(1)(c) 8.62 0.52 56.26 2.58
(1)(a) 9.75 0.50 48.02 2.34
(2)(a) 11.22 0.54 51.47 3.14
(2)(b) 11.77 0.54 54.67 3.50
(2)(c) 12.39 0.54 58.41 3.94
(2)(d) 11.72 0.54 58.06 3.70
(3)(a) 12.83 0.55 52.73 3.72
(3)(b) 12.97 0.55 53.81 3.84
(3)(c) 12.64 0.55 54.42 3.78
(3)(d) 10.48 0.53 58.45 3.25
(4)(a) 13.49 0.54 55.11 4.01
(4)(b) 12.95 0.54 45.65 3.20
(4)(c) 9.23 0.55 57.35 2.91
(4)(d) 13.18 0.57 58.43 4.36
(4)(e) 14.21 0.58 57.8 4.79
Chapter 2
Figure. 2.6 and Figure. 2.7 represent the EQE spectra and Nyquist plots for all the fabricated BHJ devices. The EQE measurements are done outside the glove box under ambient condition. From the graphs it was observed that only varying the molar mass concentration and annealing temperature, the highest obtained quantum efficiency is 55
% (Figure. 2.6a-2.6c). But with the influence of cathode buffer layer it improves to 62%
with BCP/Al as the cathode contact compared to the other (Figure. 2.6d). Also from the Nyquist plots (Figure. 2.7) it was observed that the devices with BCP/Al contact has the largest shunt resistance as compared with other device configurations and has good correlation with its photovoltaic performance. It has been reported extensively that with the variation of molar mass concentration, the annealing temperature and cathode buffer layer can also significantly improve the photovoltaic performance of organic BHJ solar cells. There are various parameters in terms of incorporation of functional organic and inorganic materials that have been mentioned in literature to improve the device performances of BHJ solar cells.30-35 In this study we have successfully explained the influence of different molar mass ratio, the annealing temperature (Tan) and the cathode buffer layer on improving the photovoltaic performance rrP3HT:PC71BM based BHJ solar cells. The results demonstrated here, are also successfully justified by the output properties of each of the cells, their thin film growth structure and energy level diagram.
These highly efficient and reproducible organic solar cells can significantly contribute to future commercialization of organic solar cell devices.