Photovoltaic devices with perovskite materials as light absorbing material were fabricated through sequential vapor deposition of lead iodide and methylammonium iodide with undoped poly3hexylthiophene (P3HT) as a hole transporting layer. The sequential vapor deposition process produced films and devices with the large grains and low defect densities, very small values of dark current, and high open circuit voltages. The thickness of the P3HT layer was a critical parameter for achieving high solar conversion efficiencies of 13.7%. The vapor deposition process also produced devices with a tight distribution of performance characteristics and very high open circuit voltages (0.99 V).

Methylammonium lead halide based perovskites have gained significant attention as a light absorbing material in recent years for applications in photovoltaic devices.1–4 These materials have shown a promising photovoltaic performance with a power conversion efficiency (PCE) of up to 19% when iodine is the halide, and5 a high open circuit voltage up to 1.4 V when Bromine is the halide.6 These light absorbing perovskites can be represented as ABX3, where A can be a CH3NH37 or NHCHNH38–11 ion, B can be Pb(ii), Sn(ii), or Cd(ii)ion, and X can be either I, Br, or Cl.12–14 These organic-inorganic hybrid perovskites are successful in photovoltaic application due to their direct bandgap nature with the band gap of ∼1.57 eV which results in excellent absorption of photons.15–17 Moreover, these materials exhibit higher diffusion length of the order of 100–1000 nm, high dielectric constant of ∼18, and high carrier mobility of 20 cm2 V−1 s−1.15–17 

The planar heterojunction perovskite solar cells devices have been reported in both p-i-n and n-i-p device configurations with comparable efficiency. In p-i-n devices, the perovskite was sandwiched between Poly (3,4-ethylenedioxythiophene) (PEDOT)/NiO2 (p-doped heterojunction)18,19 and C60/PCBM (electron transporting n-doped heterojunction layer).20,21 In n-i-p devices, n-doped ZnO or n-TiO2 has been used as an electron injecting heterojunction layer (HTL) and doped spiro-MeoTAD as a hole transporting p-type HTL.22–25 Recently, doped and undoped conjugated polymers have been reported to be successful as a p-heterojunction layer.26,27 Several conjugated polymers and small molecules have been investigated as a HTL.28,29 Poly3hexylthiophene (P3HT) is a well-studied conjugated polymeric system commonly employed as a light absorbing layer in organic solar cells.30 But, when P3HT was used as HTL in organic-inorganic hybrid solar cells, only modest PCE of up to 10.8% has been reported.31 

Perovskite thin films are formed by different processing conditions that can be broadly classified as (a) solution process and (b) vapor process. The solution processing (SP) involves either spin coating or spray processing.32 By spin coating, perovskite can be formed either from the solution containing the mixture of lead halide (PbX2) and methylammonium halide (MAX) or by sequentially depositing PbX2 and MAX from their respective solutions. In sequential deposition, MAX is deposited either by dipping the PbX2 films in the solution containing MAX or by spin coating MAX solution on top of spin-coated PbI2.33,34 Sequential deposition is advantageous over the single solution method as it yields the perovskite films free of pin holes which will adversely affect the device outcome. Chen et al. reported a sequential deposition process through a partial vapor evaporation process, where the PbI2 films were obtained by spin coating but the methylammonium iodide (MAI) was deposited by evaporating (subliming) MAI in the glove box by heating.35 As an alternative, one can sequentially vapor deposit both the Pb component and organic component and then mix them during an annealing process. In a recent paper, sequential vapor deposition was used to fabricate high performance p-i-n devices on PEDOT [(Poly (3,4-ethylenedioxythiophene) Polystyrene sulfonate)] coated ITO substrates.36 They used PbCl2 as the precursor for making perovskite devices.36 In this paper, we report on sequential vapor deposition of a complementary structure, n-i-p devices, TiO2 coated Fluorinated Tin Oxide (FTO) substrates using PbI2 as the precursor and with P3HT as the hole transport layer. The perovskite formed through the vapor process has the advantage of good reproducibility, fewer pinholes, and larger grain size and is amenable to large scale manufacturing using a roll-to-roll process.37 In this paper, we show that one can fabricate high quality n-i-p perovskite devices by sequentially evaporating PbI2 and MAI and combining it with careful growth of P3HT as HTL to achieve high efficiency perovskite solar cells, obtaining much higher voltages (0.99 V) with P3HT than achieved with liquid-based perovskite deposition processes. We also show that the thickness of P3HT is a critical parameter which controls the device performance.

The schematic representation of sequential vapor deposition of PbI2 and MAI is represented in Figure 1. PbI2 was evaporated under vacuum onto a FTO substrate coated with compact TiO2 layer.

FIG. 1.

Schematic representation of sequentially vapor deposited perovskite film in two steps: (a) evaporation of PbI2 followed by vapor assisted growth in (b) glass petri dish or (c) graphite vessel.

FIG. 1.

Schematic representation of sequentially vapor deposited perovskite film in two steps: (a) evaporation of PbI2 followed by vapor assisted growth in (b) glass petri dish or (c) graphite vessel.

Close modal

PbI2 was deposited at the rate of 1 Å/s resulting in films with thickness of around 200 nm. After this process, the films were transferred to a nitrogen glove box for MAI deposition. Two methods were used for MAI deposition. In the first, the substrate was placed over a covered petri dish with MAI sprinkled around the PbI2 substrate. The petri dish was placed over a hotplate which was heated to 180 °C (the real temperature inside the petri dish is lower, ∼150 °C) and within few minutes, the color of the substrate starts changing from yellow to dark brown indicating the formation of perovskite. The formation of perovskite was complete at the end of three hours of vapor annealing. In the second method, the substrate covered with PbI2 was placed inside a graphite container which also contained MAI powder. The container was then placed on the hot plate. Two different grades of graphite were used, a high density grade graphite and a low density grade graphite. It was found that the results were far more reproducible with high density grade graphite, because unlike low-density graphite, it does not absorb and retain residual MAI vapor, which can affect the formation of perovskites during subsequent runs. While both the petri dish and graphite techniques produced similar results for the best cells, the graphite container method was far more reproducible with better uniformity of growth than the petri dish method. This is because graphite is a conductor and prevents any static charge build up during the MAI sublimation process, unlike the petri dish method where static charge builds up and particles of MAI agglomerate and deposit on the substrate. It is interesting to note that the CH3NH3PbI3 was formed at 150 °C and it is stable at that temperature. In order to remove the excess MAI deposited over the perovskite, the substrate was kept under vacuum (1 × 10−6 torr) for periods of 24-48 h.

In Fig. 2, we show the structure of the n-i-p device.

FIG. 2.

Schematic diagram of n-i-p perovskite device deposited on compact TiO2.

FIG. 2.

Schematic diagram of n-i-p perovskite device deposited on compact TiO2.

Close modal

In Fig. 3, we show the X-ray and SEM images for a typical perovskite film produced using the sequential vapor deposition technique. The SEM image in Fig. 3(a) shows that perovskite layer is highly crystalline and continuous with the grain size of ∼500 nm. Figure 3(b) represents the XRD spectrum of the vapor deposited ∼400 nm thick perovskite films.

FIG. 3.

Secondary electron SEM image of perovskite thin films (left) and XRD spectra (right) of completely vapor grown perovskite (CH3NH3PbI3) over FTO/TiO2 planar structure. The perovskite peaks at 14.15°, 28.49°, 31.94°, and 43.25° are assigned to (110), (220), (310), and (330) planes which correspond to an orthorhombic crystal structure of CH3NH3PbI3. While the peaks at two theta values of 26.59°, 33.8°, 37.85°, 51.56°, 61.69°, and 65.65° correspond to the peak of TiO2.

FIG. 3.

Secondary electron SEM image of perovskite thin films (left) and XRD spectra (right) of completely vapor grown perovskite (CH3NH3PbI3) over FTO/TiO2 planar structure. The perovskite peaks at 14.15°, 28.49°, 31.94°, and 43.25° are assigned to (110), (220), (310), and (330) planes which correspond to an orthorhombic crystal structure of CH3NH3PbI3. While the peaks at two theta values of 26.59°, 33.8°, 37.85°, 51.56°, 61.69°, and 65.65° correspond to the peak of TiO2.

Close modal

The perovskite films thus prepared by sequential vapor deposition were coated with undoped P3HT as a p-type heterojunction HTL. P3HT was spin coated from chlorobenzene solutions with varying concentrations of P3HT. Increasing concentration of P3HT in the solution leads to increasing thickness of the P3HT film deposited on the perovskite. The device was completed by thermally evaporating 100 nm of gold over the P3HT. We deliberately used undoped P3HT, because we did not want to introduce mobile dopants which can lead to excessive hysteresis or to changes over time.

In Fig. 4(a), we show the illuminated I-V curves for our best device produced using the vapor deposition process. The corresponding quantum efficiency curve is shown in Fig. 4(b). Note from Fig. 4(a), the high open-circuit voltage (∼0.96 V) is slightly higher than the previous best voltage (0.92 V) reported using P3HT as the hole transport layer.31 We believe that the reason we consistently get a higher voltage than in previous studies31,38 is that we do not have any liquids (which could lead to shunts) remaining in the device. The absence of liquids also leads to a very reproducible process, as shown in Fig. 5. The reproducibility of the devices is shown by the chart in Fig. 5, which shows a tight distribution of voltages, currents, and fill factors over five successive runs and 28 devices in all [6 device dots per run × 5 runs, out of which 2 dots were obviously shorted and are excluded from the chart. Note that in all thin film PV research projects, one always gets occasional shorts unless one works in a Class 10 clean room with appropriate gowns and hoods to prevent dust particles from affecting the devices and all chemicals are tightly filtered. Such shorted devices should always be excluded from the histogram since they do not contribute any physics to the data set].

FIG. 4.

Illuminated I-V curves for best sequentially vapor deposited cell (a) and corresponding QE (b). The illumination intensity was 100 mW/cm2. The conversion efficiency of the device is 13.7%. The area of the cell is 0.104 cm2. Note the high open-circuit voltage (0.96 V).

FIG. 4.

Illuminated I-V curves for best sequentially vapor deposited cell (a) and corresponding QE (b). The illumination intensity was 100 mW/cm2. The conversion efficiency of the device is 13.7%. The area of the cell is 0.104 cm2. Note the high open-circuit voltage (0.96 V).

Close modal
FIG. 5.

Charts and table showing variations in device parameters of 28 devices on 5 substrates fabricated one after another using the graphite container for annealing. The primary variation is in fill factor, which we believe is due to excess resistance from residual MAI vapor left on the top of the device. Note the uniform distribution of high open circuit voltages approaching 1 V (the highest was 0.99 V) and currents.

FIG. 5.

Charts and table showing variations in device parameters of 28 devices on 5 substrates fabricated one after another using the graphite container for annealing. The primary variation is in fill factor, which we believe is due to excess resistance from residual MAI vapor left on the top of the device. Note the uniform distribution of high open circuit voltages approaching 1 V (the highest was 0.99 V) and currents.

Close modal

We also studied the hysteresis of the I-V curves. The hysteresis curve is shown in Fig. 6. Unlike almost all other hysteresis curves, our short circuit current does not change significantly upon hysteresis; rather, the fill factor decreases by ∼8%-10%.

FIG. 6.

Hysteresis curve for a vapor deposited cell. The blue curve is the original, going from Voc to Jsc. The red dotted curve is going from Jsc to Voc (with 2 min wait between points), and the black square curve is the next scan going from Voc to Jsc. There is very little change in short circuit current. The major change upon hysteresis is in fill factor. Area of the cell is 0.104 cm2. The initial efficiency (for the blue curve) was 12.6% which became 11.1% for both the red dotted and the black square curves.

FIG. 6.

Hysteresis curve for a vapor deposited cell. The blue curve is the original, going from Voc to Jsc. The red dotted curve is going from Jsc to Voc (with 2 min wait between points), and the black square curve is the next scan going from Voc to Jsc. There is very little change in short circuit current. The major change upon hysteresis is in fill factor. Area of the cell is 0.104 cm2. The initial efficiency (for the blue curve) was 12.6% which became 11.1% for both the red dotted and the black square curves.

Close modal

We discovered that the thickness of the P3HT layer was a critical parameter that determined the fill factor. The reason for this is that P3HT has a relatively high resistivity (∼104 Ω cm). This means that the resistance of the layer, and space charge limited current through it, can become a factor in determining the fill factor if the thickness is too high. If the thickness is too low, then the entire layer is depleted, and then, the work function of the electrode plays a role in determining the voltage. This situation is almost identical to what is found in amorphous Si p-i-n cells, where the p a-(Si,C) layer has a low doping, and if it is thin and totally depleted, the voltage drops. In Table I, we show the data for voltage, series resistance, and fill factor for three devices with different thicknesses of P3HT. The thickness of P3HT was measured by depositing films on glass substrates and then using a surface profileometer.

TABLE I.

Photovoltaic device performance of vapor grown perovskite films with varied thickness of P3HT as a HTL. Rsr is series resistance of the device.

Polymer concentration (mg/ml) Thickness (nm) Voc (V) Jsc (mA/cm2) Rsr (Ω cm2) FF (%) PCE (%)
17  45  0.92  15.79  13.1  61.7  9.0 
12  30  0.96  21.76  8.6  65.3  13.7 
08  20  0.88  13.04  7.8  62.5  7.2 
Polymer concentration (mg/ml) Thickness (nm) Voc (V) Jsc (mA/cm2) Rsr (Ω cm2) FF (%) PCE (%)
17  45  0.92  15.79  13.1  61.7  9.0 
12  30  0.96  21.76  8.6  65.3  13.7 
08  20  0.88  13.04  7.8  62.5  7.2 

Note from Table I, how a very thin P3HT layer leads to a loss in voltage, whereas a too thick layer leads to a loss due to increasing series resistance. Note how the series resistance for the two thicker P3HT films scales with the thickness of the film. For the thinnest film, since the film is totally depleted, as indicated by the precipitous voltage drop, the current is carried by space charge limited current, and the linear scaling factor does not apply. The best device performance was obtained with the intermediate thickness of the P3HT film. This device showed an excellent voltage (0.96 V), greater than the previous best report in the literature using undoped P3HT (0.92 V). The highest voltage we observed was 0.99 V. The corrected short circuit current (Jsc) of 21.8 mA/cm2 (estimated from external quantum efficiency measurements), open circuit voltage (Voc) of 0.96 V, and a fill factor of 65.3% lead to the PCE of ∼13.7% (Figures 4(a) and 4(b)). Note that our fill factors are much higher than the previous best report on cells using P3HT.31 We believe this improvement is due to the optimization of the thickness of P3HT layer.

In summary, we have fabricated CH3NH3PbI3-based n-i-p perovskite solar cells with sequential vapor deposition without the use of any liquids during the growth of perovskites. In addition, we demonstrate that P3HT can be an easy-to-use alternative to other more expensive p-heterojunction hole-transport materials such as doped spiro-MeOTAD and PTAA in fabrication of high efficiency perovskite solar cells. We also show that it is necessary to carefully control the thickness of P3HT so as to avoid the problems associated with total depletion and voltage loss when it is too thin, and high resistance when it is too thick.

This work was supported in part by NSF Grant Nos. ECCS-1232067 and CBET-1336134 and Iowa Energy Center.

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