Low dark current (off-current) and high photo current are both essential for a solution processed organic photodetector (OPD) to achieve high photo-responsivity. Currently, most OPDs utilize a bulk heterojunction (BHJ) photo-active layer that is prepared by the one-step deposition of a polymer:fullerene blend solution. However, the BHJ structure is the main cause of the high dark current in solution processed OPDs. It is revealed that the detectivity and spectral responsivity of the OPD can be improved by utilizing a photo-active layer consisting of an interdiffused polymer/fullerene bilayer (ID-BL). This ID-BL is prepared by the sequential solution deposition (SqD) of poly(3-hexylthiophene) (P3HT) and [6,6] phenyl C61 butyric acid methyl ester (PCBM) solutions. The ID-BL OPD is found to prevent undesirable electron injection from the hole-collecting electrode to the ID-BL photo-active layer resulting in a reduced dark current in the ID-BL OPD. Based on dark current and external quantum efficiency (EQE) analysis, the detectivity of the ID-BL OPD is determined to be 7.60 × 1011 Jones at 620 nm. This value is 3.4 times higher than that of BHJ OPDs. Furthermore, compared to BHJ OPDs, the ID-BL OPD exhibited a more consistent spectral response in the range of 400 – 660 nm.
© 2015 Optical Society of America
The basic operating principle of OPVs and OPDs is similar in terms of utilizing a heterojunction to dissociate a tightly bounded exciton. However, OPDs operate under a higher electric field and require a high on/off current ratio to have a high photo-responsivity at low light intensity. A low dark current (off-current) in conjunction with a high photo current is essential for an OPD to achieve high photo-responsivity. Currently, most OPDs utilize a bulk heterojunction (BHJ) photo-active layer prepared by the one-step deposition (OSD) of a polymer:fullerene blend solution [1–5]. The BHJ structure prepared by this OSD process has sufficiently high exciton dissociation efficiency for application in OPDs. However, undesirable charge injection from the electrode to the photo-active layer, such as electron injection from a hole collecting electrode (ITO/PEDOT) or hole injection from an electron collecting electrode (Al or Ca), are expected because both the polymer domains and fullerene domains are in contact with the same electrode. This undesirable charge injection will reduce the photo-responsivity of BHJ OPDs by increasing the dark current.
An organic bilayer structure consisting of an electron donating layer and an electron accepting layer can adequately reduce undesirable charge injection. At first, the organic bilayer structure was prepared by sequential vacuum deposition (SqVD) to produce an organic solar cell . The SqVD processed bilayer had a plain heterojunction with a small heterojunction area compared to the BHJ structure. The heterojunction area of the bilayer could be improved by the thermal annealing of the bilayer film resulting in the formation of an interdiffused bilayer (ID-BL) structure . Recently, methods to prepare a photo-active layer by the sequential solution deposition (SqD) of a polymer solution and a fullerene solution were developed [8–15]. It was found that the SqD processed photo-active layer could attain the desirable ID-BL structure (polymer/interdiffusion/fullerene) after careful optimization of the solvent and heterojunction agents . Because only one kind of layer (polymer or fullerene layer) in the ID-BL will contact the corresponding electrode, the ID-BL structure is expected to reduce the undesirable charge injection from the electrode when compared to the BHJ structure and improve the exciton dissociation efficiency compared to the planar BL structure.
In this work, we present a study on the performance of an OPD utilizing the ID-BL photo-active layer prepared by the SqD process. Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were used as the electron donating and electron accepting organic semiconductors, respectively. The photodetector parameters including on/off current ratio, responsivity, detectivity and spectral response were investigated for both the ID-BL and BHJ OPDs. The detectivity of the ID-BL OPD was greatly enhanced compared to that of the BHJ OPD due to the reduced dark current. Furthermore, the ID-BL OPD exhibited a consistent spectral response over the range of 400 – 660 nm that was not observed with the BHJ OPD.
2.1 Device fabrication
Commercial indium–tin-oxide (ITO) coated glasses (20 Ω) were cleaned in ultrasonic baths of isopropyl alcohol (IPA) (10 min) and acetone (10 min). Substrates were then dried in a convection oven for 30 min. UV/ozone treatment for 20 minutes was used to clean and remove any organic residues present on the surface. Poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) (Clevious P VP AI 4083, Germany) was spin coated on top of the clean ITO anode and dried at 110 °C for 10 minutes in a vacuum oven. The PEDOT:PSS layer was used as a hole collecting layer. Poly(3-hexylthiophene) (P3HT) (Rieke Metals Inc., USA) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) (Nano-C, USA) were used for the photo-active layer in the OPD. The photo-active layers were prepared by two different processes. One was prepared by the one-step solution deposition of a P3HT:PCBM blend solution (OSD process) containing 30 mg P3HT and 30 mg PCBM in 1 ml 1,2-dichlorobenzene (DCB) (Aldrich, USA). The other was prepared by the sequential deposition of P3HT and PCBM solutions (SqD process). For the SqD processed film, three different concentrations of P3HT solution (20, 40 or 60 mg in 1 ml DCB) were prepared. The prepared P3HT solutions were spin-coated onto the PEDOT:PSS coated ITO substrate at a speed of 800 rpm for 180 seconds followed by the deposition of a PCBM solution with a concentration of 13.9 mg PCBM in 1 ml dichloromethane at a speed of 4000 rpm for 30 seconds. OSD films were annealed at 150 °C for 10 minutes while SqD films were annealed at 150 °C for 20 minutes. All thermal annealings were conducted on a hot plate under a N2 atmosphere. The electron transporting layer was formed on the OSD or SqD films by spin-coating a TiO2 nanoparticle solution at 1500 rpm for 30 seconds followed by drying at 70 °C for 1 hour to remove the residual solvent . Finally, an electron collection electrode of aluminium (Al) was deposited onto the ITO/PEDOT:PSS/OSD or SqD layer/TiO2 by thermal evaporation through a shadow mask at a pressure of ~3 x 10−6 torr. The fabricated OPDs were encapsulated with a glass cap and a UV curable resin (Raynics, Japan) in a N2 filled glove box for further analysis.
2.2 Characterization techniques and measurements
The absorption spectra of OSD and SqD processed photo-active layers were measured using a UV-2450 ultraviolet-visible spectrophotometer (SHIMADZU, Japan). Current density vs. voltage (J-V) curves of the OPDs were measured under various light intensities using an AM 1.5G solar simulator (McScience K201 LAB50, Korea) and Keithley 2400 source meter. The external quantum efficiency (EQE) of the OPDs, which shows the photon-to-electron conversion efficiency as a function of wavelength, were obtained under short-circuit conditions with a lock-in amplifier (SR830, Stanford Research System) at a chopping frequency of 20 Hz during illumination with monochromatic light from a Xenon lamp (McScience K3100 EQX, Korea). Transient photovoltage (TPV) measurements were performed at steady state under continuous illumination from an intensity-adjustable white LED. When the device output reached a steady state, the device was perturbed using a green LED with a 1 Hz pulse rate. The resulting voltage transient was acquired using a TDS3054B Tektronix digital oscilloscope with a 1 MΩ input impedance. TPV results were fitted to a mono exponential decay function in order to find the carrier recombination lifetime .
3. Results and discussions
The chemical structures, energy band diagram and OPD device structures used in this experiment are shown in Fig. 1. Two types of photo-active layers were prepared (see experimental): One prepared by the OSD of a P3HT:PCBM blended solution resulting in the formation of a BHJ structure and the other prepared by the SqD of P3HT and PCBM solutions. Three SqD processed films were prepared using different concentrations of P3HT solution. The film prepared by the OSD process will be denoted as OSD and the SqD processed film fabricated with solutions of 20, 40 or 60 mg P3HT in 1 ml DCB will be denoted as SqD20, SqD40 and SqD60, respectively. The SqD processed P3HT/PCBM bilayer has an ID-BL structure due to the inter-diffusion of PCBM during the deposition of the PCBM solution on the P3HT bottom layer [9, 10].
In the UV-vis. absorption spectra shown in Fig. 2, both OSD and SqD films exhibited an absorption peak for P3HT in the range of 420 to 620 nm corresponding to the π-π* transition of the P3HT backbone  and an absorption peak for PCBM located around 347 nm . As shown in Fig. 2 and Table 1, the thickness of the P3HT film increases as the concentration of the P3HT solution increases. Compared to the absorption peak of P3HT in the OSD film, the peak for P3HT in the SqD60, SqD40 and SqD20 showed red-shift due to enhanced vibronic peaks appearing between 500 - 530 nm (Fig. 2(b)) . This suggests that the P3HT in the ID-BL had a highly ordered structure. Usually the highly ordered structure of the P3HT chains are not observed in a BHJ structured film that is prepared by OSD because the bulky PCBM in the film prevents the ordering of the P3HT. For the OSD film, the vibronic peak became pronounced only after thermal annealing of the film at around 150 °C due to the acceleration of the phase separation between P3HT and PCBM. When the ID-BL layer is utilized in the OPD, enhanced hole mobility in the ID-BL is expected due to the highly ordered structure of the P3HT chains [20, 21].
Semi-log J-V characteristics of OSD and SqD processed OPDs are compared in Fig. 3(a), the light on/off current ratios of the corresponding photodetectors at a bias of −1 and −3 V are plotted in Fig. 3(b) and these values are listed in Table 1 along with the shunt resistance (Rsh) values of the corresponding OPDs. The SqD40 OPD showed the highest on/off current ratio of 2.28 x 104 at −1 V followed by the SqD60, OSD and SqD20 OPDs. The on/off current ratio of the SqD OPD increased as the active layer increased in thickness and became saturated at an active layer thickness greater than ~400 nm. The SqD60 OPD showed the lowest dark current due to the thick P3HT layer (580 nm), which would suppress electron injection from the anode to the SqD60 active layer in reverse bias . Although, the SqD60 showed the lowest dark current value, the on/off current ratio of the SqD60 was lower than that of the SqD40 due to a lower photocurrent at the corresponding electric field. This was mainly ascribed to the thickness of the P3HT layer in the SqD60, 580nm which is significantly larger than the exciton diffusion length. As a result, excitons formed in the SqD60 film would recombine, instead of dissociating into electrons and holes, leading to poor photocurrent (or charge collection efficiency). Comparing the on/off current ratio of SqD40 and OSD OPDs of similar active layer thickness, the on/off current ratio of SqD40 was 8.1 and 15.4 times higher than that of the OSD at −1 V and −3 V, respectively, while showing similar photocurrents (Jph). This was mainly ascribed to the effectively suppressed dark current (Jd) of the SqD40 OPD compared to that of the OSD OPD.
The Jd is determined by the electron injection from the PEDOT:PSS layer to the lowest unoccupied molecular orbital (LUMO) of the P3HT (LUMOP3HT) or the LUMO of the PCBM (LUMOPCBM), and hole injection from the Al electrode to the highest occupied molecular orbital (HOMO) of the P3HT (HOMOP3HT) or the HOMO of the PCBM (HOMOPCBM) through the TiO2 inter-layer (Fig. 1(b)). Since the hole blocking TiO2 inter-layer effectively prevents hole injection from the Al electrode, Jd would depend mainly on electron injection from the PEDOT:PSS layer rather than on hole injection .
It is believed that the morphological difference between the SqD and OSD active layers is the main reason for the Jd difference. Since the OSD layer formed the BHJ structure, both P3HT and PCBM domains would contact the electrodes (Fig. 4(a)). The SqD20, SqD40 and SqD60 are expected to form an inter-diffused heterojunction instead of forming a planar heterojunction due to the high diffusivity of the PCBM (Fig. 4(b) and (c)). However, the PCBM in SqD20 will completely interdiffuse into the P3HT and contact the PEDOT:PSS layer (Fig. 4(b)) because the complete interdiffusion of PCBM was observed in a bilayer OPV of similar active layer thickness [9, 10]. Considering WFPEDOT – LUMOP3HT is greater than WFPEDOT – LUMOPCBM, electrons from the PEDOT:PSS layer would be mainly injected to the LUMO of the PCBM rather than the LUMO of the P3HT. Because both P3HT and PCBM domains are in contact with PEDOT:PSS layer in the OSD and SqD20 OPD, PCBM domains contacting the PEDOT:PSS layer would donate percolation paths for the electron transport to the electron collecting electrode (Al) resulting in a large Jd value for the OSD and SqD20 OPDs. In contrast, the film morphology of the SqD film having a thicker P3HT layer (SqD40 and SqD60) is better able to reduce charge recombination at the interface. The formation of a P3HT rich layer at the ITO/PEDOT:PSS would effectively block electron injection from the PEDOT:PSS layer to the photoactive layer (Fig. 4(c)). Consequently, the Jd of the SqD40 and SqD60 OPDs were greatly suppressed.
Transient photovoltage (TPV) measurements were conducted to investigate the charge recombination rate of the OSD and SqD40 OPDs. In our previous study, the TiO2 inter-layer effectively reduced the charge recombination at the interface of the photoactive layer and the Al electrode . Therefore, the differences in the carrier lifetime between the OSD and Sq40 OPDs would be ascribed mainly to the difference in the charge recombination at the PEDOT:PSS/photoactive layer interface. The total carrier density in a device at open circuit conditions under steady-state illumination could be estimated by charge extraction method . This involved integrating the total current extracted from the device immediately following switching the light off and simultaneously setting the cell to short circuit using fast solid state switches . For charge extraction measurements under open circuit conditions, the device was allowed to equilibrate under steady state bias illumination supplied by the LEDs for longer than 10 seconds . The carrier recombination lifetime (τrec) was obtained by fitting the measured photovoltage decay as a function of time with mono-exponential decay . The obtained τrec values and charge recombination rate (k) as a function of the charge carrier density are shown in Fig. 5(a) and 5(b), respectively. The τrec of the SqD40 was significantly extended in comparison to that of the OSD at the equivalent carrier density. This result indicates that the P3HT layer at the PEDOT:PSS electrode in the SqD40 OPD effectively prevented carrier recombination at the PEDOT:PSS/photo-active layer interfaces resulting in the reduced k in SqD40, which is in accordance with the result of Jd.
Figure 6 compares the EQE spectra of OSD, SqD20, SqD40 and SqD60 OPDs obtained under a bias of 0 V, −1 V and −3 V. The EQE of all the OPDs increased as the reverse bias increased. Considering the hole mobility of P3HT is ~10−4 cm2/Vs  and the electron mobility of PCBM is ~10−3 cm2/Vs , the thickness of the active layer is too thick to collect the photo-generated charge carriers without recombination under a 0 V bias. As a result, a significant non-geminate charge recombination was expected for all the OPDs under the weak electric field (0 V or forward bias condition). Applying reverse bias (−1 V and −3 V) on the OPDs would enhance the charge carrier mobility resulting in a reduction in the charge recombination of the OPDs.
The OSD OPD converted photons to electrons over a range of wavelengths from 400 nm to 630 nm, and the maximum EQE value of 60.9% under −3 V was obtained at a wavelength of 540 nm (Fig. 6(a)). The overall EQE values of the SqD60 OPD were lower than that of the OSD (Fig. 6(b)) for all of the three bias values. Interestingly, the SqD60 OPD showed a minimum EQE value at wavelength of 520 nm (the maximum absorption wavelength of P3HT) and showed a maximum EQE value at wavelengths of 400nm and 630 nm. Considering that the absorption coefficient of P3HT at wavelength of 520 nm is significantly greater than that at wavelength of 630 nm (Fig. 2) and that the thickness of the P3HT layer in the SqD60 is 580 nm, the 520 nm wavelength photon would be mostly absorbed near the PEDOT:PSS layer rather than at the P3HT/PCBM interface. The amount of excitons formed near the P3HT/PCBM heterojunction would be relatively smaller than that formed near the PEDOT:PSS layer resulting in the low exciton dissociation efficiency and the low EQE value at 520 nm. For the excitons formed by the 630 nm wavelength photons, the density of the formed excitons would be uniform throughout the P3HT layer because the P3HT has a small absorption coefficient at this wavelength. As a result, the EQE value of the SqD60 OPD at 630 nm was larger than that at 520 nm. The EQE peak at around 400 nm was mainly ascribed to the absorption of the PCBM. Since the PCBM layer was relatively thin (~70 nm), most PCBM would form a heterojunction with the P3HT by inter-diffusion into the P3HT during the SqD process, and, as a result, the excitons formed in the PCBM would be dissociated and collected more efficiently.
In contrast, the SqD40 OPD exhibited constant EQE values of ~60% in the wavelength range of 400 - 630 nm when measured under the bias of −3 V (Fig. 6(c)). The overall shape of the EQE spectrum of the SqD40 OPD at 0 V is similar to that of SqD60 at 0 V. However the EQE value of SqD40 is larger than that of SqD60. Interestingly, the shape of the EQE spectrum of SqD40 became flat as the reverse bias increased. Since the charge drift velocity (vd) is proportional to the carrier mobility (μ) and applied electric field (E) (vd = μE), the charge carrier collection efficiency at wavelengths around 550 nm could be enhanced due to the reduced charge carrier recombination under high electric field (−1 V and −3 V).
The EQE spectra of the SqD20 OPD (Fig. 6(d)) showed a broad spectrum between 400 to 600 nm, and the overall shape was similar to that of the OSD OPD suggesting that the morphology of the SqD20 film is similar with that of the OSD film with the BHJ structure. However, the EQE value of SqD20 was significantly lower that of the OSD OPD. Consequently, in terms of spectral response, the SqD40 has a more adequate structure than the OSD for OPD because the SqD40 OPD exhibited broad and constant EQE values over the wavelengths of 400 – 630 nm and exhibited similar EQE values with the OSD OPD.
Based on the light and dark J-V curves and the EQE spectrum, the responsivity and detectivity of the OPDs were obtained. The responsivities were obtained from the equation Eq. (1) and the results are shown in the Fig. 7(a) and Table 2. The R of the SqD40 OPD is 0.204, 0.227 and 0.256 A/W for the wavelength of 470, 550 and 620 nm, respectively, which is similar or higher than the R of the OSD OPD that exhibits 0.208, 0.256 and 0.218 A/W for the wavelength of 470, 550 and 620 nm, respectively. The SqD40 OPD exhibits higher responsivity for red light (620 nm) than the OSD OPD and lower responsivity for blue light (470 nm).
One of the important factors determining the performance of the photodetector is the detectivity (D*) because high detectivity can reduce the noise equivalent power (NEP) - the minimum optical power for the detector to distinguish from the noise . There are three types of noise influencing the D*: shot noise from dark current, Johnson noise and thermal fluctuation “flicker” noise [30, 31]. According to the previous reports on the OPDs, the shot noise from the dark current is considered to be the major contributor to the D* and can be expressed as
Detectivities of OPDs were calculated using Eq. (2), and their results (in Jones) are shown in Fig. 7(b) and 7(c). At a −1 V bias, the SqD40 exhibited the highest detectivity among the OPDs over the spectral range of 400 – 700 nm. The calculated detectivity of the SqD40 OPD at 620 nm was 2.40 × 1010 Jones, which was 3.4 times higher than that of the OSD OPD which was calculated to be 7.11 × 109 Jones. As shown in the Fig. 7(b) and Eq. (2), it was clear that the low dark current (noise) as well as high photocurrent (photo-responsivity) was essential to obtain high detectivity .
Compared to the OSD OPD, the SqD40 OPD showed a lower Jd value while maintaining a similar Jph with the OSD OPD resulting in the highest detectivity. As described above, the high detectivity of the SqD40 OPD is ascribed to the ID-BL film morphology (Fig. 4(c)) that can effectively reduce the dark current and enhance the heterojunction area.
In conclusion, the performance parameters of the solution processed OPD based on P3HT and PCBM were improved by adopting a photo-active structure by the SqD of P3HT and PCBM solutions. The ID-BL layer, most notably in the SqD40 OPD, hindered electron injection from the PEDOT:PSS (hole collecting electrode) to forming the P3HT rich layer contacting with PEDOT:PSS layer, thus reducing the dark current and enhancing the detectivity of the Sq40 OPD. In contrast, the OPD utilizing a BHJ photo-active layer prepared by the OSD process exhibited a large dark current compared to the SqD40 OPD. Furthermore, the SqD40 OPD exhibited a wide and consistent spectral response in the range of 400 - 660 nm compared to the OSC OPD. As a result, the detectivity of the SqD40 OPD was 3.4 times higher than that of the OSD OPD. The SqD40 OPD exhibited performance comparable to a photodetector utilizing an inorganic semiconductor. We believe that forming the ID-BL photo-active layer by the SqD process provides a novel method for reducing the noise (dark current) and improving the performance of solution processed OPDs.
This research was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (No. 2011-0031567); and by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20133030011330 and 20123010010140). We thank Dr. Kris Rathwell for the valuable discussions.
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